MQTT bridge

[Syntoxr]

What do you think about a MQTT bridge?

I think a MQTT bridge would be a great addition to the API one, as it is an easy to use and well known standard which works on most platforms.

There are two main use cases I could think of:

1. As MQTT is often used in home automation, this would be a great integration to projects like HomeAssistant or openHAB. Also, single IOT devices would be able to publish a sensor reading to a messaging platform of choice.
2. Easy to integrate, custom commands like listening to keywords and responding with automated Answers (Of course possible with the API but far easier to use for average users)

This would also be an alternative to the long polling issue, which only allows one device connected at a time, mentioned by @42wim here: #307 (comment)