Implement an option to automatically close cmus after a certain amount of time.

Probably something like:

• option idle_timer_duration int (number of seconds)
• option idle_timer_reset enum activity (reset on any interaction), action (reset on any command, i.e., anything but simple scrolling), stopped (reset while playing), none (never reset)
• option idle_timer_autostart bool (whether to start the idle timer automatically on startup)
• option idle_timer_action enum quit (exit), stop (stop playback)
• command idle-timer [-s] [duration] (reset/set the idle timer to idle_timer_duration or the arg, or stop it)
• format idle_timer (remaining time left in idle timer)