Support for idle mode

[denis-stepanov]

AdVent by default consumes almost all CPU available. It would be good to check periodically if recognition yields anything above zero level across some period (say, 10 mins) and if not, go into an idle mode by suspending all recognition attempts. Exit from this mode should be automatic by checking once a minute or so if there's new input. This feature would be especially useful when running AdVent as a daemon (see issue #7), so that one could turn off TV and then Pi could go into "calm" mode by itself. The feature should be configurable and turned on by default.