Add support for universal binaries

[wagoodman]

Today if you try to pass a universal binary to many commands it wont work:

$ quill describe /bin/ls
[0000]  INFO quill version: [not provided]
[0000] ERROR 1 error occurred:
        * unable to parse macho formatted file with blacktop: invalid magic number in record at byte 0x0

We need to unwrap each contained binary and sign accordingly (I think... more research needed here). The unwrapping / wrapping could be dealt with by https://github.com/anchore/go-macholibre/ upon detecting a universal binary without changing the overall signing/notarization logic too much.