add ability to shut down without opening the box

[foobarbecue]

As currently set up, the only way to power down the sensor is to wait for the battery to drain to 3V, the PiSugar's low-voltage disconnect point. Charging wirelessly then turns it back on. That's an OK solution but means you are doing way more discharging and charging than needed. It would be nicer to have a way to power down the sensor. Either a switch that can be triggered via a gpio, or potentially a magnetic switch that can be used by waving a magnet near the enclosure.