Generic Heater Support

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>

config/example-extras.cfg

@@ -1176,6 +1176,33 @@
 #    Directly sets the default prefix. If present, this value will override
 #    the "default_type".
 
+# The heater_generic section is used to describe a custom/generic heater.
+# These behave the same as typical heaters (extruders, heated beds) and
+# can be configured as similarly.
+# However, a gcode_id must be supplied which is used for temperature reporting.
+# See notes on the SET_HEATER_TEMPERATURE command for setting the temperature.
+#[heater_generic my_generic_heater]
+#gcode_id: C
+#    A mandatory parameter that is required for reporting the temperature
+#    through the M105 command.
+#heater_pin:
+#max_power:
+#sensor_type:
+#sensor_pin:
+#pullup_resistor:
+#adc_voltage:
+#smooth_time:
+#control:
+#pid_Kp:
+#pid_Ki:
+#pid_Kd:
+#pid_integral_max:
+#pwm_cycle_time:
+#min_extrude_temp:
+#min_temp:
+#max_temp:
+#   See the example.cfg for the definition of the above parameters.
+
 # Pause/Resume functionality with support of position capture and restore
 #[pause_resume]
 #recover_velocity: 50.