add option for diff icons in gutter

Adds an option to replace the border with icons in the right side of the gutter.
I didn't use octicons (edit: diff icons) as the diff icon's borders add a lot of visual clutter to the gutter and makes it hard to see the icon content.

I used consistent colours and shapes.
![git-diff](https://f.cloud.github.com/assets/154176/2407516/bd2489ae-aa8d-11e3-8d80-8dc2cfcbf87c.png)


Related: #7