Proviola

I think I finally managed to get Proviola working, after much hackery.  I ended up having to use the .tar.gz version, rather than the most recent soruces, and then I had to patch the .tar.gz to handle the ` .. ` notations and to un-unicodify some characters to pass to coqtop (I might have missed a few).  I made https://github.com/JasonGross/proviola-source to host my changes, and the `etc/ci/install_proviola.sh` script pulls from there.  I added `proviola` as a Makefile target because I want to be able to take advantage of make's parallelism, and it was easier that re-doing all the relevant rules from scratch in a new makefile.  The rules that I have don't support making proviola for ssreflect.  When this gets merged, I'll update the wiki to point to the proviola display as well as the coqdoc one.

This makes partial progress on #204.

Sorry for stealing this one, @spitters.