added body.plugin, composite.plugin, constraint.plugin, engine.plugin properties

src/body/Body.js

@@ -42,6 +42,7 @@ var Axes = require('../geometry/Axes');
             type: 'body',
             label: 'Body',
             parts: [],
+            plugin: {},
             angle: 0,
             vertices: Vertices.fromPath('L 0 0 L 40 0 L 40 40 L 0 40'),
             position: { x: 0, y: 0 },
@@ -719,6 +720,13 @@ var Axes = require('../geometry/Axes');
      * @type body[]
      */
 
+    /**
+     * An object reserved for storing plugin-specific properties.
+     *
+     * @property plugin
+     * @type {}
+     */
+
     /**
      * A self reference if the body is _not_ a part of another body.
      * Otherwise this is a reference to the body that this is a part of.

src/body/Composite.js

@@ -35,7 +35,8 @@ var Body = require('./Body');
             bodies: [], 
             constraints: [], 
             composites: [],
-            label: 'Composite'
+            label: 'Composite',
+            plugin: {}
         }, options);
     };
 
@@ -655,4 +656,11 @@ var Body = require('./Body');
      * @default []
      */
 
+    /**
+     * An object reserved for storing plugin-specific properties.
+     *
+     * @property plugin
+     * @type {}
+     */
+
 })();

src/constraint/Constraint.js

@@ -73,6 +73,7 @@ var Common = require('../core/Common');
         constraint.angularStiffness = constraint.angularStiffness || 0;
         constraint.angleA = constraint.bodyA ? constraint.bodyA.angle : constraint.angleA;
         constraint.angleB = constraint.bodyB ? constraint.bodyB.angle : constraint.angleB;
+        constraint.plugin = {};
 
         return constraint;
     };
@@ -391,4 +392,11 @@ var Common = require('../core/Common');
      * @type number
      */
 
+    /**
+     * An object reserved for storing plugin-specific properties.
+     *
+     * @property plugin
+     * @type {}
+     */
+
 })();

src/core/Engine.js

@@ -51,6 +51,7 @@ var Body = require('../body/Body');
             constraintIterations: 2,
             enableSleeping: false,
             events: [],
+            plugin: {},
             timing: {
                 timestamp: 0,
                 timeScale: 1
@@ -491,4 +492,11 @@ var Body = require('../body/Body');
      * @default a Matter.World instance
      */
 
+    /**
+     * An object reserved for storing plugin-specific properties.
+     *
+     * @property plugin
+     * @type {}
+     */
+
 })();