cli: Always wait for the player process to exit.

The “--player-no-close” option would previously let the player continue to
run when Livestreamer exited. This meant that control could return to the
shell with the player still running in a background process. When
Livestreamer has finished sending the stream, it now blocks until the player
exits.

This avoids awkward behaviours with M Player and MPV, which do not work well
in some cases when running as a background process. I noticed that if their
standard input was _not_ used to stream the media (e.g. “--player-http”
mode), then they would change the terminal settings after Livestreamer
exited. This would interfere with Bash and Readline.

Apparently also fixes #234, avoiding a Screen session from terminating too
early.