add attack_ignore feature (thx ShadowOfHassen for idea)

api.txt

@@ -111,6 +111,7 @@ functions needed for the mob to work properly which contains the following:
    'attack_animals'      when true mob will attack animals.
    'attack_npcs'         when true mob will attack npcs within range.
    'attack_players'      when true mob will attack players nearby.
+   'attack_ignore'       string or table of mob names not to attack.
    'owner_loyal'         when true non-docile tamed mobs attack anything player
                          punches when nearby.
    'group_attack'        when true has same mob type grouping together to attack