add support for multi-audio-track YT-videos

[a-tze]

After years of waiting, YT finally has support for multiple audio tracks as it seems.

I have no idea if this feature is limited to some "high roller" accounts or if anyone can use it, and just stumbled upon it recently:

https://youtu.be/84uelyKOMmI?t=10

Supporting this could make the whole YT publishing a lot easier - but probably needs ways to advertise the feature inside the video, e.g. with a different intro or YT title something like that.