Add support for OCI labels on image/bundle push

[phenixblue]

This adds the ability for users to specify OCI labels that will be added to the generated OCI artifact (bundle/image) during a push operation.

Fixes #153

Example:

$ imgpkg push -b index.docker.io/jmsearcy/example-bundle:v1.0.3 -l foo.bar=baz -f ../../tmp/imgpkg-test

The resulting OCI artifact can be inspected as such to show the labels added:

$ regctl image inspect jmsearcy/example-bundle:v1.0.3

{
  "created": "0001-01-01T00:00:00Z",
  "architecture": "",
  "os": "",
  "config": {
    "Labels": {
      "dev.carvel.imgpkg.bundle": "true",
      "foo.bar": "baz",
    }
  },
  "rootfs": {
    "type": "layers",
    "diff_ids": [
      "sha256:f77fc57aa0878a70576e6b45c74ce5a7070219687b92fe073f632d71b7b107f8"
    ]
  },
  "history": [
    {
      "created": "0001-01-01T00:00:00Z",
      "created_by": "imgpkg",
      "author": "imgpkg"
    }
  ]
}

[phenixblue]

Related to #412

[joaopapereira]

The change looks good to me. Added some questions and suggestions.

[joaopapereira]

LGTM