Add vim-airline for testing

I'm trying out vim-airline (https://github.com/bling/vim-airline).  I
may revert this later, if I decide that I don't like it.
Fix `git diff` output colors

After switching to the Solarized color scheme
(https://github.com/altercation/solarized), I don't need to override
these colors. The solarized ones already look good.
Add some basic documentation to linkup script