Airline: Configure to Look More Like Powerline

I like the speed and lack of dependencies of vim-airline but I prefer the look of powerline. I added a theme "powerlineish" to vim-airline that mimics the colors of classic powerline. This commit enables that theme and enables the use of powerline fonts to get the nice separators.

I wasn't sure if this should go in `.vimrc` in the status line section or in `.vimrc.bundles`. I opted for the latter so the variables are only set if you're actually using that bundle. Additionally I considered adding a configuration variable to enable the powerline fonts but ultimately decided against it.