Add expanded gist syntax

This add the ability for a gist in the YAML fron matter to designate a specific file in a gist not just the entire thing.

    ---
    gist: expalmer/4bc6203dc2a3f7134e78 sukima/510438:load_config.rb
    ---

By appending a `:filename` to the list it will be parsed to the following url:

    https://gist.github.com/sukima/510438.json?file=load_config.rb

You reference the gist in the document the same way:

    gist:sukima/510438:load_config.rb