FR: add support for oci-path images

[dmivankov]

Currently docker-archive is supported, but oci-path isn't

It'd be nice to support it for use in combination with Bazel and rules_oci.
As a reference, trivy supports this format https://github.com/aquasecurity/trivy/blob/69a5fa18ca86ff7e5206abacf98732d46c000c7a/pkg/fanal/image/oci.go#L12 (though underlying libs to work with images may be different)

[yashahuja31]

Put together a pr for the thing in 325. The input flag checks if the path given is a directory now and uses oci path for that or treats files as the usual docker tarball. Works with bazel rules oci and skopeo it seems.

Tests got added for the layers compressed and uncompressed. Path fixes for the different operating systems too. And the readme got an update. I might have missed something small but overall it should be good. Let me know your thoughts when you get a chance.