support new nodes after setup

[esdeboer]

I'm implementing homie for crownstones (a bluetooth switch/dimmer/presence detector/power monitor)
It works with a bluetooth mesh network to which you can add new nodes. It would be nice if those new nodes also show up in MQTT.

However I can't do that now as we can't subscribe to properties after setup, but I don't see anything in the spec which would disallow this.