⚠️ Breaking Change !

This release moves to more robust handling of missing MTL files by changing the fields of the Material struct to all be Option<T>: https://github.com/Twinklebear/tobj/blob/master/src/lib.rs#L574-L613 instead of default values. Previously, these default values may have been incorrectly interpreted as the values of a missing Material, instead of the material data being missing.