Highlight GitHub style fenced code blocks

One needs to opt-in to this by specifying which file types should be
highlighted:

    let g:markdown_github_languages = ['ruby', 'erb=eruby']

This may not work for all file types.

Closes #17.
Merge commit 'b2697b0adfe5428b86c8470a5f9d8da667bb2785' into develop

Conflicts:
	syntax/markdown.vim