Secure by default

We should consider `options.secure` being enabled by default. This means that obj2gltf will not attempt to load files that are outside the obj's directory.

From @OmarShehata in https://github.com/AnalyticalGraphicsInc/obj2gltf/pull/219

>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](https://en.wikipedia.org/wiki/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`.