firmware_retraction: Implementation of tuneable G10/G11 firmware retraction (#1617)

This supports getting and setting of retraction parameters without
having to restart klippy.

Signed-off-by: Len Trigg <lenbok@gmail.com>

config/example-extras.cfg

@@ -1593,6 +1593,27 @@
 #measurement_delay: 100
 
 
+# Firmware filament retraction. This enables G10 (retract) and G11
+# (unretract) GCODE commands issued by many slicers. The parameters
+# below provide startup defaults, although the values can be adjusted
+# via the SET_RETRACTION command, allowing per-filament settings and
+# runtime tuning.
+#[firmware_retraction]
+#retract_length: 0
+#   The length of filament (in mm) to retract when G10 is activated, and to
+#   unretract when G11 is activated (but see unretract_extra_length below).
+#   The default is 0 mm.
+#retract_speed: 20
+#   The speed of retraction, in mm/s. The default is 20 mm/s.
+#unretract_extra_length: 0
+#   The length (in mm) of *additional* filament to add when unretracting.
+#unretract_speed: 10
+#   The speed of unretraction, in mm/s. The default is 10 mm/s.
+#z_hop: 0
+#   The amount of lift applied to the Z axis (in mm) on retract and
+#   restored on unretract. The default is 0 mm.
+
+
 # Include file support. One may include additional config file from
 # the main printer config file. Wildcards may also be used (eg,
 # "configs/*.cfg").