Build a framework for a maintainable combat plugin

* Uses a hierarchy of WeaponClasses -> Weapons for performing attacks,
  with WeaponClass having a set of styles and associated Attacks for
  those styles. Weapons and their classes are built with an easy to use
  and clean DSL.

* Adds a BonusContainer mixin, so that Equipment, Weapons, and
  WeaponClasses can all have their own set of bonuses which apply to the
  player.

* Allows attacks to be queued to the Mobs CombatState instance from
  external code, allowing e.g., NPCs or auto-cast to queue attacks to be
  executed.

data/plugins/combat/combat.rb

@@ -0,0 +1,12 @@
+on :message, :npc_action do |player, message|
+  player_combat_state        = get_combat_state player
+  player_combat_state.target = $world.npc_repository.get message.index
+
+  unless player.attacking
+    player.start_action CombatAction.new(player)
+  end
+end
+
+on :message, :player_action do |player, message|
+  
+end