crane: Support pushing multiple image tarball

[lbuttorff]

I have a team that moves image bundles between systems often. We have been using crane to pull a list of images into a single bundle and that works great. Then we take that bundle somewhere, docker load it and docker push each image. While this works, it is not as clean as our bundling with crane. The enhancement I am requesting here is that like pulling multiple images into one tarball, crane would support pushing multiple images from a tarball to a registry.