Support more Unity shaders

[AurL]

The exporter currently supports Standard (metal) and Standard (Specular setup), but it could also support other shaders.

The property naming is almost consistent between them, so the exporter should support them as much as possible.

Nice remark from @essimoon, Additive materials could also be supported. Sketchfab could parse them and automatically set materials to additive

[sonygod]

sounds great ,
because unity standard shader have lots of limit to use,
such like occlusion could not assigned for Uv2...and so on
you mean support custom shader or something else?