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. :)
Please break this up into smaller snippets; it's hard to read this way.