Adding support for multi file gists.

Hey moski! I added a couple of lines to support multi file gists. This is super useful when you want to have one gist per blog post, and separate each code snippet in a different file.

To use, add the optional data-file attribute, like this:

<div class="gistLoad" data-id="4982341" data-file="gistfile2.cs" id="gist-4982341">Loading ....</div><br />

I am already using this on my blog here: http://silentkraken.blogspot.com

All of the snippets there belong to the same gist, they are different files.

Please merge :)