Attack entities in area/zone.

[Freagarach]

As also noted by wraitii, it would be nice if one could let entities (mostly archers) attack entities within a zone (complementary to attacking a piece of ground #1777). Meaning that the user issues such an order and the entities pick a random entitiy inside that zone to attack. Note this would _not_ really work with melee entities, since they might try to go through the ranks of the enemy to attack. But for ranged entities it could reduce the amount of "overkill", i.e. the wasted arrows on dead people.