Feature Request: add support for local network sensors

[gtoderici]

Currently the code only supports public sensors (i.e., those listed on the map).

It would be nice to add support for private sensors (i.e., those which people don't list on the map because they're indoor sensors, so there's little reason to make the readings public). This could be done in two ways: (1) by providing the local IP of the sensor and retrieving the JSON file directly; or (b) through the API provided by purple air.