Add vapoursynth to mpv formula

This would fix #33.
I'm not experienced with homebrew formula at all, so feel free to reject and do it in a better way, but I wanted to propose this nevertheless.

l.86-90 makes the `pkgconfig` path of python3 visible to the build. The way it is done was suggested by a homebrew maintainer so I hope it's fine.
l.10 uses `'python3'` instead of `:python3` because I guess l.86-90 might work only with a homebrew installed python3.