Handle mtl and texture absolute paths

[lilleyse]

Fixes #213

I confirmed that the tests fail if path.resolve is changed back to path.join.

[OmarShehata]

This works well. My only comment here is I think we should consider setting --secure to be true by default. In general, we should always create tools that are secure by default.

I don't think that should be a big issue for users of this library, since I imagine OBJs with relative absolute paths aren't common, and getting the old behavior would just be a matter of setting --secure false.

[lilleyse]

I opened a separate issue for secure by default: #220

Is this one ok to merge?

[OmarShehata]

Yup, this is good! Thanks for opening the issue.