github_flavored_markdown: Improve diff highlighting.

This PR tracks progress of this new feature. It's still in development and not ready to merge.

- Add detailed diff highlighting, apply only for -1,+1 blocks (GitHub-style) for now.

Current results can be seen [here](https://github.com/shurcooL/go/commit/c98697dd352f9e22d4d9213c97adf5f73b6733c5#commitcomment-6986624)