Add RDM Personality support

[peternewman]

Closes #29

Blocked behind #37 (builds on the changes in there).

Obviously this adds new fields to the RDMINIT struct, currently in a way that isn't very backwards compatible, so I don't know what we want to do about that in terms of future releases.

There are also some other bits it would make sense to add while making a breaking change such as:

• Manufacturer ID
• Device part of the UID (or effectively combine these two to cover the whole thing)
• Product category
• Software version/software version label
• Possibly default device label

[peternewman]

Thanks for the review too @chrisstaite !

[midikidi]

Nice work Peter I will check it out

[grahamrobertslx]

How about adding sensor data to the RDM like adding ability to add a DHT22 for temperature sensor data. PID SENSOR_VALUE