Add register map files (--map device.yaml)

[19bk]

Support named register maps so you can see meaningful names instead of raw addresses:

# device.yaml
registers:
  temperature:
    address: 40001
    type: holding
    scale: 0.1
    unit: "C"
  pressure:
    address: 40002
    type: holding
    unit: "mbar"

Then:

modbus read 192.168.1.10 --map device.yaml

Would show "temperature: 23.7 C" instead of "40001: 237".

This is a bigger feature but very useful for anyone working with specific devices.