Introduce snowball_vulnerable armor group

mods/ENTITIES/mcl_mobs/api.lua

@@ -2876,7 +2876,14 @@ local mob_activate = function(self, staticdata, def, dtime)
 	self.path.stuck_timer = 0 -- if stuck for too long search for path
 
 	-- mob defaults
-	self.object:set_armor_groups({immortal = 1, fleshy = self.armor})
+	local armor
+	if type(self.armor) == "table" then
+		armor = table.copy(self.armor)
+		armor.immortal = 1
+	else
+		armor = {immortal=1, fleshy = self.armor}
+	end
+	self.object:set_armor_groups(armor)
 	self.old_y = self.object:get_pos().y
 	self.old_health = self.health
 	self.sounds.distance = self.sounds.distance or 10

mods/ENTITIES/mcl_mobs/api.txt

@@ -35,8 +35,12 @@ functions needed for the mob to work properly which contains the following:
    'breathes_in_water' If true, mob loses breath when not in water. Otherwise,
                    mob loses breath when inside a node with `drowning` attribute
                    set (default: false).
-   'armor'         holds strength of mob, 100 is normal, lower is more powerful
-                   and needs more hits and better weapons to kill.
+   'armor'         entity armor groups (see lua_api.txt). If table, a list of
+                   armor groups like for entities. If number, set value of
+                   'fleshy' armor group only.
+                   Note: The 'immortal=1' armor group will automatically be added
+                   since this mod handles health and damage manually.
+                   Default: 100 (mob will take full dmg from 'fleshy' hits)
    'passive'       when true allows animals to defend themselves when hit,
                    otherwise they amble onwards.
    'walk_velocity' is the speed that your mob can walk around.