Load markdown files through markdown-with-front-matter-loader

Created by: pugnascotia

This PR adds support for loading *.md files through markdown-with-front-matter-loader. This makes it simple to load and render Markdown through CRA.

Testing

1. Create a demo app with ./tasks/cra.sh my-app-test
2. cd my-app-test
3. Create src/_posts/hello-world.md

---
title: 'Hello world'
---
# Hello world

This is a test

```javascript
function() { }
```

4. Modify src/App.js to load the file

import post from './_pages/hello-world.md';

5. Include the post in the page:

<p className="App-intro">
  To get started, edit <code>src/App.js</code> and save to reload.
</p>
<div dangerouslySetInnerHTML={{__html: post.__content}}></div>

6. Start dev mode and check the page renders as expected.

yarn start

7. Build a production bundle:

yarn run build

8. Check that the production build works as expected by serving it and browsing http://localhost/

docker run -it --rm -v $PWD/build:/usr/share/nginx/html -p 80:80 nginx:alpine