[Feature Suggestion - twitch.tv] Enable Audio_Only format

[simonics]

Adding the request parameter 'allow_audio_only': 'true' alongside the two instances of 'allow_source': 'true' in the twitch extractor module appears to enable an Audio_Only format, which is regularly restricted to the twitch.tv mobile app.

After implementing this change, youtube-dl -F twitch.tv/[channel] on a random channel returns the following additional format summary:

Audio_Only   mp4        audio only  128k , mp4a.40.2

Would this be worth adding in? I'd imagine that the format might be useful in low-bandwidth situations.

[buchno]

Would this be worth adding in? I'd imagine that the format might be useful in low-bandwidth situations.

Would be great for the music channels.

[bcongdon]

Addressed in #8786 #8791.

[dstftw]

audio only formats will be supported in the next version.