Make *simple* format selection

[rasguanabana]

Now it uses unpredictable youtube-dl format, which isn't very user friendly. Use -a (bestaudio), -v (bestvideo) and -av (best) to provide streamable defaults (unless -S option is selected, then -av will select bestvideo+bestaudio for merge).

Always choose bestaudio for audio only (do we need audio format selection?)

For video, let user specify height and then choose the nearest format. Prefer streamable ones, but if -av is specified and user chose high quality (e.g. 1080), then use DASH formats.

Also, make -av default instead of -a alone.