multimedia/py-knowit: New port: Know better your media files

KnowIt can use MediaInfo, ffprobe (FFmpeg) or mkvmerge (MKVToolNix)
It supports MKV regardless if MediaInfo, FFmpeg or MKVToolNix are installed.

WWW: https://github.com/ratoaq2/knowit

This port is a dependency of multimedia/py-subliminal port update (wip).