Allow custom formats and make mp3 conversion optional

[shantanugoel]

Scratching an itch for download part that I have for some time:

1. MP3 conversion is not always desired since it may take a long time for big playlists and is also sometimes not possible on all systems (hate the ugly warning messages for missing ffmpeg for those cases ;))
2. bestaudio downloads webm most of the time from youtube but most of the non-pc audio players (software as well as hw) can't play it, making mp3 a necessity. Added a way to use custom format strings for such cases (e.g. -f 'bestaudio[ext=m4a]/mp4' will download all songs in m4a natively which can be played by most players and is a pretty good quality format to boot).

[SathyaBhat]

Would skip conversion to mp3 as an optional thing be better for most? ie have MP3 conversion as default and pass params to skip conversion?

[shantanugoel]

Yeah, that seems a reasonable default since we also have "bestaudio" as default which may download webm and thus needs conversion to mp3. I will make the appropriate change to keep conversion as default and provide option to skip.

[SathyaBhat]

Awesome, will wait for the change before merging in. thank you :) 👍

[shantanugoel]

Added the change.

[SathyaBhat]

Thanks so much!