MQTT improvements

[mzealey]

• The "+" symbol in heater+ caused mosquitto to reject all messages
• Create plain number values with type specified in the topic name for easier parsing
• Add in flags to detect eg low battery issues

[Juerd]

Hi,

Thanks for your PR (and the other one which is already merged)!

Sorry, I'm kinda opinionated about the new topics that duplicate existing functionality with a slightly different presentation, so that's not happening... One reason I'm strict about this, is that I and many others just throw everything that comes in over MQTT into a time series database. I don't want duplication there :). Splitting on " " isn't hard, and in several languages you can easily just parse the string as a number directly (e.g. javascript parseFloat) so if your language doesn't do that that's just missing functionality over there. I'd accept a boolean environment variable to change the format, or a configurable format template, but would rather not send the same values twice.

I like the other changes though.