Option/extension for draco point compression

[pjcozzi]

See KhronosGroup/glTF#874

[pjcozzi]

Quick stats: 10-25x better than gzip. Decode 110 MB in JavaScript in 1 second.

[ggetz]

For b3dm and i3dm, this should be handled by the gltf extension itself. Probably mention in a non-normative section that the extension is available.

Point clouds and draco would target post 1.0