Use GitHub Flavored Markdown for parsing the markdown

It would really help if we could use GitHub Flavored Markdown. Especially for code snippets.

Since Kramdown uses Coderay for highlighting stuff, it's really hard to customize the highlighting colours (it adds `style="color:..."` to DOM nodes... not nice to override).

Using GFM really just sets the language attribute on DOM nodes. Syntax highlighting can then be more easily done with front-end libraries like Prism, Rainbow, Highlight.js...