Add support for GitGutter and Critic Markup

I love using Afterglow but I was frustrated with the lack of GitGutter colours and Critic Markup so I added them in. Because the styles aren't detrimental to others I thought it should be part of the main package. Works especially well with the proposed changes by @felixhao28 to Markdown Editing (waiting for the PR to be accepted there). Screenshot to see the outcome.

![critic markup and gitgutter](https://cloud.githubusercontent.com/assets/7970809/4593398/aab4dd48-5085-11e4-820a-fabdea84227b.png)