Add support for *.glb files

[emackey]

We should add some form of support for saving and loading *.glb, the binary form of glTF. Currently, attempting to load a *.glb file results in a mostly blank screen with a terse message:

The file will not be displayed in the editor because it is either binary, very large or uses an unsupported text encoding.

Can an extension replace this message with its own representation of the text portion of a binary document? Can the text be editable?

@HowardWolosky Do you have any ideas about how we could solve this? I'm starting to think that in the long run, .glb will end up being much more widely adopted than plain-text .gltf.

[emackey]

Export added in #35. Import still wanted.

[emackey]

Implemented in #44. 🎉