Add support for tavdog/tidbyt-mqtt

[DouweM]

https://github.com/tavdog/tidbyt-mqtt

Similar to #9, this would remove the dependency on Tidbyt cloud. It would be easier to implement (with a new target-tidbyt-mqqt) as we can still push individual images, instead of having to manage our own app rotation. I'm not sure tidbyt-mqtt actually has app rotation fully implemented, though, and it's definitely not easier to set up than tidbyt/hdk.