Emit engine-init event at the very end of the constructor

This allows plugins to perform actions which require all plugins to be
initialized (for example, calling showChunk which requires the mesher
and stitcher and shader to have loaded, and set in the game instance).

index.js

@@ -187,7 +187,7 @@ function Game(opts) {
 
   this.cameraPlugin = plugins.get('game-shell-fps-camera') // TODO: support other plugins implementing same API
 
-
+  this.emit('engine-init', this);
 }
 
 inherits(Game, EventEmitter)