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. :)
Also, looking at the [org-mode manual](http://orgmode.org/manual/Structure-of-code-blocks.html) I don't see that `HEADER` directive being used for code blocks. What does that add?