Create a describe command for a bundle

**Describe the problem/challenge you have**
As a user I want to know the contents of a bundle before pulling it completely down. Specifically what's really important is to know the images in the bundle (and dependent bundles) as when I copy (relocate) a whole bundle (and images) I would want to know what happened and why.

**Describe the solution you'd like**
I would want a command like:

`imgpkg describe -b my-bundle` that will print the following details:

- Content of the bundle
- Images in the bundle (original references, current references and strategy that lead to the new name if applicable. Related to https://github.com/vmware-tanzu/carvel-community/pull/22)

This way a user could do some diagnostics to determine the provenance of a bundle or to know what he'll need to plan (in terms of a CopyConfig file) to do proper image copying.