Add onItemStart event to LoadingManager

This could possibly be changed to an ```onLoadingStarted``` event and only fire if we are not already loading.
I added it to the end because I didn't want to break any existing code. After a quick ```git grep``` it looks like no one has ever passed anything into the LoadingManager constructor.

I'll rework the order to
```
THREE.LoadingManager = function ( onStart, onLoad, onProgress, onError)
```