Allow gcode commands with preheat options (#274)

* Allow gcode commands with preheat options

I use this to run a heatsoak macro during preheat.
  It moves to bed center, maxes the part cooling fan,
  and starts my nevermore chamber filter to circulate
  more air.

* Add documentation for preheat gcode

ks_includes/config.py

@@ -406,6 +406,7 @@ class KlipperScreenConfig:
         item = {
             "extruder": cfg.getint("extruder", 0),
             "bed": cfg.getint("bed", 0),
-            "heater_generic": cfg.getint("heater_generic", 0)
+            "heater_generic": cfg.getint("heater_generic", 0),
+            "gcode": cfg.get("gcode", None)
         }
         return item