Added support to options particleNoiseTex and particleSpriteTex. In this way it is possible to load a texture before and use it always in  all instances.  

Dunno if you will agree with this pull request, but the reason to create it is because I'm using a lot of GPUParticleSystem instances and I need to improve all the requests to textures that this component is doing. 

Now I just need to do: 
```js 
// .... 
var textureLoader = new THREE.TextureLoader();
var particleNoiseTex = textureLoader.load("images/perlin-512.png");
var particleSpriteTex = textureLoader.load("images/particle2.png");

// ... 
// each time I need to instance GPUParticleSystem 

	var particleSystem = new THREE.GPUParticleSystem({
			maxParticles: 200,
			particleNoiseTex: particleNoiseTex, 
   			particleSpriteTex: particleSpriteTex
		});
// .... 
```