Do not rely on global threejs

[wolfmanfx]

No description provided.

[mattdesl]

Thanks but this is a design decision, see here:
#9 (comment)

As soon as you require ThreeJS the bundle size will go way up. Not everybody is using ThreeJS via npm and the chances of using the same version is pretty low (it changes so frequently). This is why it's a peer dependency instead of a regular dependency.