Handle image manifest lists media type

[ixmatus]

We said we accepted the image manifest list media type when requesting manifests for a docker image. Docker distribution recently started honoring that, which breaks hocker-manifest because hocker-manifest doesn't actually know what to do with that result (hence c37f317).

This ticket tracks the work to support the image manifest list media type which provides a list of platform-specific references to image manifests for a docker image.