Add phong shading

[andeplane]

Current shading is very simple and not so pretty, so we should add phong shading.

If we want to enable bond rendering between atoms, we will need the ray traced shading like we have in Atomify. The only problem is that three.js doesn't let us override vertexPosition in the fragment shader for shading, so we need to add an issue / create a PR so this is supported.

For version 1.0 we can simply just use the lighting three.js has and interpolate vertexPosition the regular way.