Added missing physics property to GameConfig

The typescript typings for GameConfig are missing the physics property, so I added it.

Author: mkimmet

src/boot/Config.js

@@ -98,6 +98,7 @@ var ValueToColor = require('../display/color/ValueToColor');
  * @property {object} [images] - [description]
  * @property {string} [images.default] - [description]
  * @property {string} [images.missing] - [description]
+ * @property {object} [physics=null] - [description]
  */
 
 /**