Use default directional lights in pythreejs backend

[bdice]

I'm all for setting up a default lighting scheme for the pythreejs backend in the name of getting something decent drawn on the screen with a minimal amount of code (so the light feature isn't necessary in this case). Default lights were added in the vispy backend in 33d7b81 if you want to use the same directions and relative intensities.

Originally posted by @klarh in #11 (comment)