Add syntax highlighting for vimdiffs

The colors for vimdiffs were ugly, so I made them pretty, to use the already existing colors from the theme.

Before:

![screen shot 2013-12-12 at 16 18 29](https://f.cloud.github.com/assets/795488/1734036/bd3a3db0-6340-11e3-8f50-3c9fe89e627c.png)

After:

![screen shot 2013-12-12 at 16 19 24](https://f.cloud.github.com/assets/795488/1734041/cd303efe-6340-11e3-922b-c6b2473d6e12.png)