Add support for listening to MQTT as data source

[cryptaliagy]

Using MQTT is likely to provide better network performance than HTTP, which would directly translate to better battery life for devices that are being powered off of external batteries. By adding an MQTT client data source,

This could be done in 2 ways:

• Set up a separate MQTT listener output (docker container/etc)
• Add configuration options for enabling MQTT alongside the Pixy server

Since the Pixy server has a small memory footprint, I am more inclined to use the second option. This comes with the additional benefit that it allows the existing publish process to continue without having to add any variants or multiple Docker tags per version

[cryptaliagy]

Following #11 and #14 , this can now be safely implemented