gcode: Support running arbitrary gcode on extruder change

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example.cfg

@@ -150,6 +150,18 @@ filament_diameter: 3.500
 #   otherwise cause retraction followed immediately by pressure
 #   buildup. This setting only applies if pressure_advance is
 #   non-zero. The default is 0.010 (10 milliseconds).
+#deactivate_gcode:
+#   A list of G-Code commands (one per line) to execute on a G-Code
+#   tool change command (eg, "T1") that deactivates this extruder and
+#   activates some other extruder. It only makes sense to define this
+#   section on multi-extruder printers. The default is to not run any
+#   special G-Code commands on deactivation.
+#activate_gcode:
+#   A list of G-Code commands (one per line) to execute on a G-Code
+#   tool change command (eg, "T0") that activates this extruder. It
+#   only makes sense to define this section on multi-extruder
+#   printers. The default is to not run any special G-Code commands on
+#   activation.
 #
 # The remaining variables describe the extruder heater.
 heater_pin: ar4