github_flavored_markdown: Improve diff highlighting.

Add detailed diff highlighting, apply only for -1,+1 blocks
(GitHub-style) for now.
github_flavored_markdown: Apply detailed diff to all blocks.

Update to latest github.com/sourcegraph/syntaxhighlight API.
For diffs, highlight "\ No newline at end of file" lines correctly.
Merge pull request #6 from shurcooL/feature/detailed-diff-highlighting

github_flavored_markdown: Improve diff highlighting.