Support upload of VP9

[maflcko]

Youtube started deleting all VP8 transcodes greater than 360p a year ago. Right now it seems about 80 percent [1] of Videos are transcoded to high resolution VP9 transcodes. youtube2mediawiki should support the upload of the highest available resolution.

For example:

$ youtube-dl -F 9BUlsHYhSVA | grep webm
171         webm      audio only  DASH audio , audio@ 48k (worst)
242         webm      240p        DASH video , video only
243         webm      360p        DASH video , video only
244         webm      480p        DASH video , video only
247         webm      720p        DASH video , video only
248         webm      1080p       DASH video , video only
43          webm      640x360    <--- The version which gets uploaded by default

The problem is that audio and video streams are separated. Though, they can easily be merged by

$ ffmpeg -i dash-video -i dash-audio -c:v copy -c:a copy outFile.webm

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[1] Non-representative test: There are 29 videos on https://www.youtube.com/user/firefoxchannel/videos of which 23 are transcoded into WebM DASH. (23 / 29 = 0.793103448275862)