Support loading models

[davepagurek]

Here's sort of how it works now:

• Add your obj and mtl into the public/models directory (e.g. public/models/monkey.obj and public/models/monkey.mtl)
• Load a model in your code with `loadObj(objName, mtlName):

  const monkey = loadObj('monkey', 'monkey');

• Use attachModel to add it to a point the same way you would add geometry (it returns the root node, which you can use to scale/position the whole model)

  .define('leaf', (root) => {
  const model = root.attachModel(monkey);
  model.scale(Math.random() * 0.5 + 0.5);
  })