Add Powerline fonts

I switched to the version of Ubuntu Mono that has been patched for
Powerline in d39f783. I'm adding Lokaltog/powerline-fonts as a submodule
so that I'll have the font readily available.
Remove an unused setting

When I switched from vim-powerline to Powerline in d39f783, I was
playing with fonts a little. I neglected to remove the line setting the
font to PragmataPro. It doesn't really matter since the line was
commented out at the time, but I'd rather get rid of it anyway.