cartesian: Initial support for dual carriages

Add support for additional carriages on cartesian printers.  This is
used by some printers to handle multiple extruders.

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

config/example-extras.cfg

@@ -98,6 +98,30 @@
 #   activation.
 
 
+# Support for cartesian printers with dual carriages on a single
+# axis. The active carriage is set via the SET_DUAL_CARRIAGE extended
+# g-code command. The "SET_DUAL_CARRIAGE CARRIAGE=1" command will
+# activate the carriage defined in this section (CARRIAGE=0 will
+# return activation to the primary carriage). Dual carriage support is
+# typically combined with extra extruders - use the SET_DUAL_CARRIAGE
+# command in the activate_gcode / deactivate_gcode section of the
+# appropriate extruder. Be sure to also use that mechanism to park the
+# carriages during deactivation.
+#[dual_carriage]
+#axis:
+#   The axis this extra carriage is on (either x or y). This parameter
+#   must be provided.
+#step_pin:
+#dir_pin:
+#enable_pin:
+#step_distance:
+#endstop_pin:
+#position_endstop:
+#position_min:
+#position_max:
+#   See the example.cfg for the definition of the above parameters.
+
+
 # Multi-stepper axes. On a cartesian style printer, the stepper
 # controlling a given axis may have additional config blocks defining
 # steppers that should be stepped in concert with the primary