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

website/docs/advanced-plugins.md

@@ -93,6 +93,21 @@ module.exports = {
          */
         remarkPlugins: [],
         rehypePlugins: [],
+        /**
+         * Truncate marker, can be a regex or string.
+         */
+        truncateMarker: /<!--\s*(truncate)\s*-->/
+         /**
+         * Blog feed
+         * If feedOptions is undefined, no rss feed will be generated
+         */
+        feedOptions: {
+          type: '', // required. 'rss' | 'feed' | 'all'
+          title: '', // default to siteConfig.title
+          description: '', // default to  `${siteConfig.title} Blog`
+          copyright: '',
+          language: undefined; // possible values: http://www.w3.org/TR/REC-html40/struct/dirlang.html#langcodes
+        };
       },
     ],
   ],