Add YouCompleteMe (and supertab in a supporting role)

I'd like to give YouCompleteMe another chance. Now that I have UltiSnips
set up the way I like it, I think that YouCompleteMe will help me work
faster by surfacing snippets that I didn't know I had or ones that I
forgot about.

To get this all working together well, I brought in supertab, thanks to
the guidance posted in this Stack Overflow answer:

  http://stackoverflow.com/a/22253548/18986