Adds the ability to register a custom/generic heater through a new extra called heater_generic.
This takes a gcode_id that is reported by M105. Adds a new command to set the temperature for any heater using the syntax:
SET_HEATER_TEMPERATURE HEATER=<heater_name> TARGET=<target_temp>
Signed-off-by: John Jardine <john@gprime.net>
John Jardine