Add mobs as an object type

* Account for mobs: movement, look command
* Add health and player.take_damage placeholder
Close range combat

* No real system yet. Players turn into corpses

Minor:
* Fix "look at mob" command
* Fix few PEP8 errors
Add targeting (key "t")

* Targets closest player, doesn't account for visibility yet. Player is
marked as "X" if it is being targeted. Target does not move with the
player yet
Player can shoot at other players

* Hit "t" to target and "f" to shoot other player
Merge branch 'combat'