Add support for adding multiple WiFis

[alvarolb]

The thing.add_wifi interface should be able to support more than one Wifi Connection.

[georgevbsantiago]

Hi, @alvarolb

Any proposals on this issue?

It would be very interesting to have this functionality integrated into the Thinger-Arduino library.

I saw a proposal on WifiManager's Github that might serve as inspiration: https://github.com/esp8266/Arduino/issues/2698