OBJLoader: 2 materials on same mesh creates 2 meshes

Hi there, 

It's nor really a bug but it would be nice to have a bool for that.

the objLoader creates one mesh per material. Could we make that optional? In some cases people might only be interested in getting the correct geometry.

https://github.com/BabylonJS/Babylon.js/blob/master/loaders/OBJ/babylon.objFileLoader.js#L596

Thanks!