Extract Org-specific code blocks. Resolves gollum#256.

`gsub` org-specific code blocks in the code filter so that a block's contents won't be subjected to tag filtering. See https://github.com/gollum/gollum/issues/256. It's late, so I'll leave this open and would appreciate a quick glance. :)
> `src_<language>[<header arguments>]{<body>}` Are we supporting that syntax?

I can't get that syntax to work using gollum with `org-ruby`, so I think we might as well let that pass. If `org-ruby` ever implements it, we can too.