use highlight-js for code block elixir syntax highlighting

@josevalim Here are some examples how it looks like:

![Example 1](http://i.imgur.com/iWnPj7P.png)

![Example 2](http://i.imgur.com/RlCrUzL.png)

![Example 3](http://i.imgur.com/FZCY5QU.png)

![Example 4](http://i.imgur.com/NafdwS2.png)