Improvment of diff

This pull request is the fixed version for : https://github.com/chriskempson/base16-builder/pull/27

**What is the pull request for**
On light and dark theme, add three dict entries :
- "+" lines are stylized in green
- "-" lines are stylized in red
- "!" lines are stylized in purple

Cross repository, it also fix https://github.com/chriskempson/base16-textmate/pull/7

Here are the screenshots for the base-16 theme "default", dark and light
![dark](https://f.cloud.github.com/assets/752030/297893/8fd4cd5a-954a-11e2-81cc-750f8cee2f64.png)
![light](https://f.cloud.github.com/assets/752030/297894/903194f4-954a-11e2-910e-9749d084c92b.png)