Fix highlight broken by code fence nested in block quote

[rhysd]

Fix #207 Fix #190

This PR adds syntax highlighting for code fences nested in block quotes.

> ```
> some code
> ```

Currently the highlighting is broken since vim-markdown assumes that ``` code fences are at the start of lines.

This fixes the highlight as follows:

Before	After

I also added the small clean up at 20c58c1. If you don't want it, please let me know. I'll remove the commit.