add support for installing fonts

I used a different naming scheme for the example cask, prefixing it with `font-` (`font-dejavu.rb`)  I think some name collisions are inevitable otherwise.

It would also be possible to link just the directory `dejavu-fonts-ttf-2.34` into `~/Library/Fonts` instead of linking each individual font. I don't know if you would consider that to be preferable.