Allow gltf tree postprocessing during export
Allow gltf tree postprocessing during export

This allows the user of the library to add custom properties (for example, GLTF extensions to materials) during export. I am using this to add the https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_unlit extension to materials containing the string "unlit" in their name, but this is only one example.
mikedh/trimesh,mikedh/trimesh,mikedh/trimesh,mikedh/trimesh