Add support for Specular and IOR material extensions

[jrz371]

Specular
https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_materials_specular/README.md

IOR
https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_materials_ior/README.md