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

[jrm]

Summary

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

Test Plan

portlint -C: OK
testport: OK (poudriere: 10.3-RELEASE-p10, i386)
testport: OK (poudriere: 10.3-RELEASE-p10, amd64)
testport: OK (poudriere: 11.0-RELEASE-p1, i386)
testport: OK (poudriere: 11.0-RELEASE-p1, amd64)

[jrm]

Add a bit more information to pkg-descr.