Added custom on_step for mob arrows (thx npxcoot)

api.lua

@@ -2473,7 +2473,7 @@ function mobs:register_arrow(name, def)
 		switch = 0,
 		arrow = true,
 
-		on_step = function(self, dtime)
+		on_step = def.on_step or function(self, dtime)
 
 			self.timer = self.timer + 1
 

api.txt

@@ -170,6 +170,7 @@ This function registers a arrow for mobs with the attack type shoot.
             the parameters are (self, pos, node)
         'tail' when set to 1 adds a trail or tail to mob arrows
         'tail_texture' texture string used for above effect
+    'on_step' is a custom function when arrow is active, nil for default.
 
 
 mobs:register_egg(name, description, background, addegg)