`power` command should use `ONOFF_BOOL` param type

[jwiese]

Ah, it indeed does not accept on or off. The proper fix would be to introduce a new type by extending the https://github.com/pallets/click/blob/master/click/types.py#L344 to check first if on or off is handled. This type should also be used directly in power command, but that can be left for another PR :-)

Originally posted by @rytilahti in #42

ONOFF_BOOL is now in main.py