Introduce cgltf_write.h
Introduce cgltf_write.h

This writes out nicely-formatted JSON and ignores externally-referenced
buffer data.

Does not yet support sparse accessors, skinning, or morph targets, but
it's a good start and I've been using it successfully for a couple weeks
using several test models.

Fixes #52

jkuhlmann/cgltf,jkuhlmann/cgltf,jkuhlmann/cgltf