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. :)
Does this work if the same line also contains [switches and header arguments](http://orgmode.org/manual/Structure-of-code-blocks.html)? Seems to me that `$4` would contain that information too with the current regexp.