Add MQTT interface

[denis-stepanov]

From documentation:

On a final note, while direct communication with Telegram from ESP8266 is possible, it can be clearly seen that it is not the best choice. It blows up the sketch size and puts a lot of load on ESP (especially when Internet or Telegram become unresponsive). Probably, a better option would be to send a message to an MQTT server in a local network, and have some application on top to talk to Telegram. This might be done in future versions of the program.

This ticket is to make a design and first implementation.