feat(v2): Implement plugin creating feed for blog posts (#1916)

* feat(v2): Implement feed for blog posts

Fixes: #1698

Test plan:
- added tests

Ran `yarn build` on website with the following config (and disabled blog
from preset-classic):

```js
[
'@docusaurus/plugin-content-blog',
  {
    path: '../website-1.x/blog',
    feedOptions: {
      copyright: 'Copy',
      type: 'atom',
    },
  },
],
```
which genereted the following feed:
```xml
<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
    <id>https://v2.docusaurus.io/blog</id>
    <title>Docusaurus Blog</title>
    <updated>2018-12-14T00:00:00.000Z</updated>
    <generator>https://github.com/jpmonette/feed</generator>
    <link rel="alternate" href="https://v2.docusaurus.io/blog"/>
    <subtitle>Docusaurus Blog</subtitle>
    <icon>https://v2.docusaurus.io/img/docusaurus.ico</icon>
    <rights>Copy</rights>
    <entry>
        <title type="html"><![CDATA[Happy 1st Birthday Slash!]]></title>
        <id>Happy 1st Birthday Slash!</id>
        <link href="https://v2.docusaurus.io/blog/2018/12/14/Happy-First-Birthday-Slash"/>
        <updated>2018-12-14T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[![First Birthday Slash](/img/docusaurus-slash-first-birthday.svg)]]></summary>
    </entry>
    <entry>
        <title type="html"><![CDATA[Towards Docusaurus 2]]></title>
        <id>Towards Docusaurus 2</id>
        <link href="https://v2.docusaurus.io/blog/2018/09/11/Towards-Docusaurus-2"/>
        <updated>2018-09-11T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[Docusaurus was [officially announced](https://docusaurus.io/blog/2017/12/14/introducing-docusaurus) over nine months ago as a way to easily build open source documentation websites. Since then, it has amassed over 8,600 GitHub Stars, and is used by many popular open source projects such as [React Native](https://facebook.github.io/react-native/), [Babel](https://babeljs.io/), [Jest](https://jestjs.io/), [Reason](https://reasonml.github.io/) and [Prettier](https://prettier.io/).]]></summary>
    </entry>
    <entry>
        <title type="html"><![CDATA[How I Converted Profilo to Docusaurus in Under 2 Hours]]></title>
        <id>How I Converted Profilo to Docusaurus in Under 2 Hours</id>
        <link href="https://v2.docusaurus.io/blog/2018/04/30/How-I-Converted-Profilo-To-Docusaurus"/>
        <updated>2018-04-30T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[> _“Joel and I were discussing having a website and how it would have been great to launch with it. So I challenged myself to add Docusaurus support. It took just over an hour and a half. I'm going to send you a PR with the addition so you can take a look and see if you like it. Your workflow for adding docs wouldn't be much different from editing those markdown files.”_]]></summary>
    </entry>
    <entry>
        <title type="html"><![CDATA[Introducing Docusaurus]]></title>
        <id>Introducing Docusaurus</id>
        <link href="https://v2.docusaurus.io/blog/2017/12/14/introducing-docusaurus"/>
        <updated>2017-12-14T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[![Introducing Slash](/img/slash-introducing.svg)]]></summary>
    </entry>
</feed>
```

* new feedOptions type 'all' and use correct path

packages/docusaurus-plugin-content-blog/src/blogUtils.ts

@@ -0,0 +1,147 @@
+import fs from 'fs-extra';
+import globby from 'globby';
+import path from 'path';
+import {Feed} from 'feed';
+import {PluginOptions, BlogPost, DateLink} from './types';
+import {parse, normalizeUrl} from '@docusaurus/utils';
+import {LoadContext} from '@docusaurus/types';
+
+export function truncate(fileString: string, truncateMarker: RegExp | string) {
+  const truncated =
+    typeof truncateMarker === 'string'
+      ? fileString.includes(truncateMarker)
+      : truncateMarker.test(fileString);
+  return truncated ? fileString.split(truncateMarker)[0] : fileString;
+}
+
+// YYYY-MM-DD-{name}.mdx?
+// prefer named capture, but old node version do not support
+const FILENAME_PATTERN = /^(\d{4}-\d{1,2}-\d{1,2})-?(.*?).mdx?$/;
+
+function toUrl({date, link}: DateLink) {
+  return `${date
+    .toISOString()
+    .substring(0, '2019-01-01'.length)
+    .replace(/-/g, '/')}/${link}`;
+}
+
+export async function generateBlogFeed(
+  context: LoadContext,
+  options: PluginOptions,
+) {
+  if (!options.feedOptions) {
+    throw new Error(
+      'Invalid options - `feedOptions` is not expected to be null.',
+    );
+  }
+  const {siteDir, siteConfig} = context;
+  const contentPath = path.resolve(siteDir, options.path);
+  const blogPosts = await generateBlogPosts(contentPath, context, options);
+  if (blogPosts == null) {
+    return null;
+  }
+
+  const {feedOptions, routeBasePath} = options;
+  const {url: siteUrl, title, favicon} = siteConfig;
+  const blogBaseUrl = normalizeUrl([siteUrl, routeBasePath]);
+
+  const updated =
+    (blogPosts[0] && blogPosts[0].metadata.date) ||
+    new Date('2015-10-25T16:29:00.000-07:00');
+
+  const feed = new Feed({
+    id: blogBaseUrl,
+    title: feedOptions.title || `${title} Blog`,
+    updated,
+    language: feedOptions.language,
+    link: blogBaseUrl,
+    description: feedOptions.description || `${siteConfig.title} Blog`,
+    favicon: normalizeUrl([siteUrl, favicon]),
+    copyright: feedOptions.copyright,
+  });
+
+  blogPosts.forEach(post => {
+    const {
+      id,
+      metadata: {title, permalink, date, description},
+    } = post;
+    feed.addItem({
+      title,
+      id: id,
+      link: normalizeUrl([siteUrl, permalink]),
+      date,
+      description,
+    });
+  });
+
+  return feed;
+}
+
+export async function generateBlogPosts(
+  blogDir: string,
+  {siteConfig, siteDir}: LoadContext,
+  options: PluginOptions,
+) {
+  const {include, routeBasePath} = options;
+
+  if (!fs.existsSync(blogDir)) {
+    return null;
+  }
+
+  const {baseUrl = ''} = siteConfig;
+  const blogFiles = await globby(include, {
+    cwd: blogDir,
+  });
+
+  const blogPosts: BlogPost[] = [];
+
+  await Promise.all(
+    blogFiles.map(async (relativeSource: string) => {
+      // Cannot use path.join() as it resolves '../' and removes the '@site'. Let webpack loader resolve it.
+      const source = path.join(blogDir, relativeSource);
+      const aliasedSource = `@site/${path.relative(siteDir, source)}`;
+      const blogFileName = path.basename(relativeSource);
+
+      const fileString = await fs.readFile(source, 'utf-8');
+      const {frontMatter, excerpt} = parse(fileString);
+
+      let date;
+      // extract date and title from filename
+      const match = blogFileName.match(FILENAME_PATTERN);
+      let linkName = blogFileName.replace(/\.mdx?$/, '');
+      if (match) {
+        const [, dateString, name] = match;
+        date = new Date(dateString);
+        linkName = name;
+      }
+      // prefer usedefined date
+      if (frontMatter.date) {
+        date = new Date(frontMatter.date);
+      }
+      // use file create time for blog
+      date = date || (await fs.stat(source)).birthtime;
+      frontMatter.title = frontMatter.title || linkName;
+
+      blogPosts.push({
+        id: frontMatter.id || frontMatter.title,
+        metadata: {
+          permalink: normalizeUrl([
+            baseUrl,
+            routeBasePath,
+            frontMatter.id || toUrl({date, link: linkName}),
+          ]),
+          source: aliasedSource,
+          description: frontMatter.description || excerpt,
+          date,
+          tags: frontMatter.tags,
+          title: frontMatter.title,
+        },
+      });
+    }),
+  );
+  blogPosts.sort(
+    (a, b) => b.metadata.date.getTime() - a.metadata.date.getTime(),
+  );
+
+  return blogPosts;
+}