Optional MQTT support/bridge for virtual devices

[dxdc]

@suderman I was wondering what your thoughts were on incorporating MQTT support for interaction with ISY variables.

I don't think it would be a lot more work necessarily to incorporate, and it would allow for a lot of flexibility. Essentially, an MQTT bridge between ISY variables and non-ISY variables.

For example, instead of hb as the prefix, it could be hbmqtt. Then, it could be used to synchronize variable states between MQTT and ISY.

There have been other attempts to do this (e.g., https://github.com/berndpfrommer/ISY-Homie-Bridge) as well, but I think direct integration through MQTT would allow for a lot of power and flexibility esp. when considering how Node-RED can interact with MQTT.

Using the homie convention directly would be really interesting, if it could mirror all ISY states of course, but even just using the variables would be quite powerful.

Here is one implementation of MQTT in Node:
https://github.com/arachnetech/homebridge-mqttthing/blob/master/index.js