cli: Add a --best-stream-default option.

This makes 'best' the default stream if no stream
is specified.

Resolves #349.
cli: Make stream output order when specifying multiple options more sane.

The priority is now:

1. File output via -o/--output or -O/--stdout
2. --player-passthrough
3. --player-continuous-http
4. --player-fifo
5. --player-http
plugins.azubutv: Handle beta.azubu.tv.