Add anchor link for titles in documents

[zcbenz]

It is useful to use link with hash to reference a certain API in the documents, like this:
https://github.com/atom/electron/blob/master/docs/api/tray.md#traysethighlightmodehighlight

[jlord]

I added the id option to the site's markdown parser, 1c24753, so it now adds an id. But looking at the example you mentioned, the id is a bit awkward for methods:

<h3 id="tray.sethighlightmode(highlight)">Tray.setHighlightMode(highlight)</h3>

Also, that id doesn't even work as an anchor 😞

We are using redcarpet as the parser because it worked best with highlighting (I think), but I will try and experiment with another so that these work.

[jlord]

When GitHub Pages updates the version of redcarpet it uses our ids will get sorted 🍕

Things in motion: github/pages-gem#143

[jlord]

The parser has been updated so our docs now have the ids: http://electron.atom.io/docs/v0.30.0/api/tray/#tray-sethighlightmode-highlight

I just dug around and found the way to get the headers to be links in Jekyll as well... PR coming 🔜