duffle inspect should accept digests

[bacongobbler]

See #307 for duffle inspect

In #307, duffle inspect accepts a repository name as input. It should probably accept a digest as well.

For example:

$ duffle build .
...
==> Successfully built bundle helloworld:a5d8b894e648ba3dbe9553ded3f133fe75f02b86
$ duffle bundles
NAME                            VERSION                                         DIGEST
hub.cnlabs.io/helloworld        1.0.0                                           556fa7c5dd67621200414571ebeb9f3dc5d9f0da
$ duffle inspect hub.cnlabs.io/helloworld:1.0.0
-----BEGIN PGP SIGNED MESSAGE-----
...

But this does not work:

$ duffle inspect 556fa7c5dd67621200414571ebeb9f3dc5d9f0da
Error: no bundle name found