New port, multimedia/av: Programmable Ruby interface for FFMPEG/Libav

Adding this because it is a dependency of
multimedia/rubygem-paperclip-av-transcoder, which in turn is a dependency
of the upcoming www/mastodon port.

WWW: https://github.com/ruby-av/av

Approved by: swills (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D10347