Rewrite YouTube.choose_best as YouTube.ch_best
The library no longer expects the media scripts to set both 'default'
and 'best' media streams. Instead, only the 'best' quality stream is set
if there are >1 streams available. The media script is still responsible
to determine the best media stream.
NOTES:
- Format IDs are now "media stream format IDs" ("fmt_id") which are used
to identify the media streams (when there are >1)
- If there is only one media stream available, it is not necessary to
set any media stream format IDs ("fmt_id")
- default stream is whatever the media script sets as the first stream
in the returned qargs.streams
- best is now a flag (boolean)