vid.stab formula and ffmpeg formula update

Adds a formula for [vid.stab](https://github.com/georgmartius/vid.stab) ([homepage](http://public.hronopik.de/vid.stab/)).

Extends the ffmpeg formula by adding an option to compile ffmpeg with `--enable-libvidstab`, as described in the [ffmpeg docs](http://ffmpeg.org/ffmpeg-filters.html#vidstabdetect).