this currently uses a sketchy but apparently working workaround,
which will be removed once the neccessary changes in youtube-dl
are implemented
Fixes#1277
It's passed with the '--format' option to youtube-dl.
If it isn't set, we don't pass '--format best' so that youtube-dl can
use the options from its configuration file.
Signed-off-by: wm4 <wm4@nowhere>