* 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"><]]></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">< 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"><]]></summary>
</entry>
</feed>
```
* new feedOptions type 'all' and use correct path
