Add obj, glb and gltf mesh types to URDFLoader

[cognmbs]

Hey, first I would like to thank you for your great package, I saw in your demo that you are also supporting obj, gltf and glb mesh files. demo link: https://gkjohnson.github.io/urdf-loaders/javascript/example/bundle/ Can you add them to this package as well?

[gkjohnson]

You're able to add custom file format loaders with the exising API - see in the docs.