add matroska media type #51

Thank's alot for amazing program!
I'm interested to add .mkv file format and had a high compression by no lose quality!
like this:
ffmpeg -i input.mp4 -strict strict -map 0 -q:v 2 -c:v libx265 -preset slow -s 1280:720 -c:a copy -scodec copy output.mkv
please add more type of converter media.