New option to wait for the player process to exit

This avoids an awkward behaviour with M Player and MPV, which no not work
well in some cases when running as a background process.

The “--player-no-close” option lets the player continue to run while
Livestreamer exits. This means that control of the CLI terminal is returned
to the shell, while the player runs in a background process. If the standard
input of M Player or MPV was not used to stream the media (e.g. the
“--player-http” mode was used instead), then the player would change the
terminal settings after Livestreamer exited. This would interfere with the
settings set up by Bash and Readline.