Add a plugin which allows creating mixins for Mobs

Adds a mob-extension plugin, which allows creating modules as mixins to
be monkey-patched into the Mob class.  Having a standard method of doing
this prevents confusing problems with mixin name conflicts, by allowing
the plugin to raise an error that 2 mixins with the same name exist.

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

@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<plugin>
+    <id>mob-extension</id>
+    <version>0.1</version>
+    <name>Mob Extensions</name>
+    <description>Adds support for extending Mobs with new functionality</description>
+    <authors>
+        <author>Gary Tierney</author>
+    </authors>
+    <scripts>
+        <script>extension.rb</script>
+    </scripts>
+    <dependencies>
+    </dependencies>
+</plugin>