MQTT.read_messages doesn't work properly with json messages

[DeadCat93]

Json is returned truncated to the first comma. It seems to be because of using split(",") in line 547 in mqtt.py.

[electricalgorithm]

It is a reasonable request. I had this problem when I was working on the project, but haven't had time to provide a proper solution back then. The quick fix we had there with splitting according to "," character was a fast-patch but made it unreliable. I believe it is possible (and reasonable) to track down quotation marks and/or brackets with a simple "tokenizer" implementation to at least give people enough freedom to use commas in quotation marks.

Are there any efforts on this @selengalp?