Add support for player / npc following

Implements a new MobExtension plugin which adds 'follow', and 'chase'
mixins that allow the mob to follow behind another mob, and chase them
while keeping at a safe distance to fire projectiles.  Also adds a new
public method 'raycast' to CollisionManager, for drawing a line through
the world using bresenham's line algorithm whille checking for any impenetrable
objects.

data/plugins/entity/mob/following/following.rb

@@ -0,0 +1,58 @@
+on :message, :walk do |player, msg|
+  player.reset_interacting_mob
+end
+
+on :message, :player_action do |player, msg|
+  ## todo: need a better way of mapping option numbers to their purpose
+  player.follow($world.player_repository.get(msg.index)) if msg.option == 3
+end
+
+##
+# A <code>MobExtension</code> for making a <code>Mob</code> trail behind, or chase a given mob.
+module FollowingMobExtension
+  ##
+  # Follow a mob and trail behind them.
+  def follow(mob)
+    do_follow(self, mob, behind: true)
+  end
+
+  ##
+  # Chase a mob (with the intention of getting in front of them), also optionally
+  # stopping within a projectile distance when at a position where a projectile can
+  # reach the target.
+  def chase(mob, projectile_distance: nil)
+    do_follow(self, mob, front: true, projectile_distance: projectile_distance)
+  end
+end
+
+MobExtension::register(FollowingMobExtension)
+
+private
+
+def do_follow(source, mob, behind: false, front: false, projectile_distance: nil)
+  source.interacting_mob = mob
+
+  schedule 0, true do |task|
+    # stop the task unless we're still interacting with the other mob.
+    unless self.interacting_mob.eql? mob
+      task.stop
+      next
+    end
+
+    next unless source.walking_queue.size <= 1
+
+    distance = mob.position.get_distance(source.position)
+
+    unless projectile_distance.nil?
+      next if distance <= projectile_distance &&
+        $world.collision_manager.raycast(source.position, mob.position)
+    end
+
+    if distance > 15
+      reset_interacting_mob
+    end
+
+    walk_to(mob, behind: behind, front: front)
+  end
+end
+

data/plugins/entity/mob/following/plugin.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0"?>
+<plugin>
+  <id>mob-following</id>
+  <version>1</version>
+  <name>Following</name>
+  <description>Adds following for mobs.</description>
+  <authors>
+    <author>Steve Soltys</author>
+  </authors>
+  <scripts>
+      <script>following.rb</script>
+  </scripts>
+  <dependencies>
+    <dependency>mob-walk-to</dependency>
+  </dependencies>
+</plugin>