Send state: off mqtt messages

[neilenns]

Add support for sending state off messages via MQTT. A timer should get set/reset whenever motion is detected and after the timer expires a simple state: off message gets sent to the topic.

The delay before this gets sent should be configurable. Setting it to 0 should turn it off. Default should be something reasonable like 30 seconds or a minute.

[neilenns]

This will also require a map to keep track of the timers for each of the different sent MQTT messages. Probably do a map based on the MQTT topic is easiest.