Shriekling/doc

• Shrieklings do what kids do best: run around fast and scream. At least they are fragile.
• Killing shrieklings will inflict a morale penalty, so killing lots of them at once is not recommended. If you do not care about loot, lure them into a building and burn it down, killing the zombies inside without any morale penalty. Alternatively, running over them with a vehicle also works. The Psychopath trait also gets rid of the morale penalty.
• Eventually if you kill enough of them (100) the penalty goes away. And the penalty slowly reduces in number the more you kill
• Can shriek for a lot of Noise, just hope their big brother isn't around.
• Added in 0.D (Danny).