Add a wifi coverage heat map

[glance-]

According to some news sources irobot roomba software will start to generate wifi coverage maps: https://www.theverge.com/2018/1/3/16845006/roomba-wifi-map-home-coverage-irobot

This library could use the reported wifistat to make equivalent wifi maps, based on the wifistat events.

I would love to sea a heatmap, both over rssi and snr.