Ability to find external files and textures

[mramato]

I have models where the obj paths are incorrect or absolute but the files themselves are all there and located in the same directory as the obj file. It would be great if we either:

• Always looked in the same directory as the obj file if the initial texture/mtl/etc.. isn't where the model says it should be.

or

• Allowed an array of directories to be passed to the converter and it would search those directories for the file is not found in the obj specified location.

The first one is probably easier and I'm fine with that until we have a good use case for the more robust feature.