Add diff colors

I don't know if you're interested but the commit adds some color when using a diff with Sublime Text (for example, using [SublimeGit](https://sublimegit.net/))

Before:
![from](https://cloud.githubusercontent.com/assets/692077/3406474/dc1bb3f0-fd8f-11e3-918b-b06b0d7fb1a6.png)

After:
![to](https://cloud.githubusercontent.com/assets/692077/3406475/e65b459c-fd8f-11e3-8f40-820720b9126b.png)

Great theme!