Describe the issue

tinyobjloader does not assign material(and materialId) when no mtllib or mtl reader callback supplied.
But Blender does: #383 (comment)

It'd be better to add option to assign material and materialId when usemtl exists, but no mtllib or mtl reader callback supplied

To Reproduce

See the PR #383 and discussion here(reproducible test file there)
#383 (comment)

Expected behavior

Add option to assign material and materialId when usemtl exists.

Environment

• TinyObjLoader version: release