Configurable light color

[UstymUkhman]

Hey! Awesome plugin, really <3 it. I've noticed that the white color for DirectionalLights is hardcoded right here, so I'd like to suggest to make it configurable from data params we're passing to the constructor for the CSM initialization.

As a workaround I'm currently using this hack:

this.csm = new CSM({ ... });

this.csm.lights.forEach(light =>
  light.color.set(0xffff00)
);

But I don't think that's ideal, what do you think about that?