syntax highlighting in github commit message boxes

[nedtwigg]

GitHub does some syntax highlighting when rendering commit messages, we should match its behavior whenever a user is typing into a commit textarea.

Hardest part is just that GitHub uses a monospace font, different than all of the other textareas we are enhancing. If we finish

• modify or replace OverType #99

then implementing this would be pretty easy.

Some important fact-finding we need to do first: what aspects of markdown does GitHub render?