Add metallic/smoothness map slot to custom model import#852

Allowing a metallic(R)/smoothness(A) map would allow much better quality texturing for all types of objects, and without having to build an assetbundle and possibly script an object to behave like an existing object type(coin, chip, etc.).
We are already using the normal/bump slot which is optional, the functionality is already there for custom maps.
If the map is not specified it would default to the material tab options.
Attached is a metal/wood chip using an assetbundle, but that has no functionality.
This is not possible with current custom model import options.
I could make a reference on creating the map correctly.