add mobs_animal.eat_grass_block setting

cow.lua

@@ -1,6 +1,15 @@
 
 local S = minetest.get_translator("mobs_animal")
 
+-- should cows eat grass blocks and mess up the environment?
+
+local eat_gb = minetest.settings:get_bool("mobs_animal.eat_grass_block") ~= false
+local replace_what = { {"group:grass", "air", 0} }
+
+if eat_gb then
+	table.insert(replace_what, {"default:dirt_with_grass", "default:dirt", -1})
+end
+
 -- Cow by sirrobzeroone
 
 mobs:register_mob("mobs_animal:cow", {
@@ -51,10 +60,7 @@ mobs:register_mob("mobs_animal:cow", {
 	},
 	view_range = 8,
 	replace_rate = 10,
-	replace_what = {
+	replace_what = replace_what,
-		{"group:grass", "air", 0},
-		{"default:dirt_with_grass", "default:dirt", -1}
-	},
 --[[
 	pick_up = {"default:grass_1", "default:dry_grass_1"},
 	on_pick_up = function(self, entity)

settingtypes.txt

@@ -9,3 +9,5 @@ mobs_animal.rat (Enable Rat) bool true
 mobs_animal.sheep (Enable Sheep) bool true
 mobs_animal.warthog (Enable Warthog) bool true
 mobs_animal.hairball (Enable Kitten Hairball drops) bool true
+
+mobs_animal.eat_grass_block (Enable Cow/Sheep eating grass blocks) bool true

sheep.lua

@@ -4,6 +4,15 @@
 local S = minetest.get_translator("mobs_animal")
 local random = math.random
 
+-- should sheep eat grass blocks and mess up the environment?
+
+local eat_gb = minetest.settings:get_bool("mobs_animal.eat_grass_block") ~= false
+local replace_what = { {"group:grass", "air", -1} }
+
+if eat_gb then
+	table.insert(replace_what, {"default:dirt_with_grass", "default:dirt", -2})
+end
+
 -- sheep colour table
 
 local all_colours = {
@@ -115,10 +124,7 @@ for _, col in ipairs(all_colours) do
 		},
 		view_range = 8,
 		replace_rate = 10,
-		replace_what = {
+		replace_what = replace_what,
-			{"group:grass", "air", -1},
-			{"default:dirt_with_grass", "default:dirt", -2}
-		},
 		fear_height = 3,
 
 		on_replace = function(self, pos, oldnode, newnode)