added 'mob_active_limit' to limit mobs in game

settingtypes.txt

@@ -28,5 +28,8 @@ mob_chance_multiplier (Mob chance multiplier) float 1.0
 #    When false Mob no longer drop items when killed
 mobs_drop_items (Mob drops) bool true
 
-#....Sets minimum distance around player that mobs cannot spawn
+#    Sets minimum distance around player that mobs cannot spawn
 mob_nospawn_range (Mob no-spawn range) float 12.0
+
+#    Sets maximum number of active mobs in game (0 for unlimited)
+mob_active_limit (Mob Active Limit) float 0