Different handling of separate textures

`obg2gltf` has a `writer` callback option that gives the user control of how separate textures are saved. It might be better for `obj2gltf` to return an object containing the gltf or glb and a dictionary of separate textures, similar to what [gltf-pipeline](https://github.com/AnalyticalGraphicsInc/gltf-pipeline/blob/master/lib/processGltf.js#L68-L70) does.

This would be a breaking change.