Use voxel-texture for texturing and different textured voxel meshes.

This will allow voxels within voxel meshes to have different sided textures. Materials can be specified as such:

```js
// first is block with grass on top
// dirt on the bottom and grass_dirt on the sides
// the other blocks have the same texture on all sides
materials: [
  ['grass', 'dirt', 'grass_dirt'],
  'brick', 'dirt', 'obsidian', 'crate'
]
```

The various shorthand ways to set the faces of the texture can be viewed here: https://github.com/shama/voxel-texture

The only issue now is the front and left sides have their textures rotated 90deg. Looking into a fix now that will land is voxel-texture@0.2.1.

Thanks!