add manifest command

Enable inspection (aka "shallow pull") of images' manifest info, and
also the creation of manifest lists (aka "fat manifests").

The workflow for creating a manifest list will be:

`docker manifest create new-list-ref-name image-ref [image-ref...]`
`docker manifest annotate new-list-ref-name image-ref --os linux --arch
arm`
`docker manifest push new-list-ref-name`

The annotate step is optional. Most architectures are fine by default.

There is also a `manifest inspect` command to allow for a "shallow pull"
of an image's manifest: `docker manifest inspect
manifest-or-manifest_list`.

To be more in line with the existing external manifest tool, there is
also a `-v` option for inspect that will show information depending on
what the reference maps to (list or single manifest).

Signed-off-by: Christy Perez <christy@linux.vnet.ibm.com>
Signed-off-by: Daniel Nephin <dnephin@docker.com>

cli/command/manifest/testdata/inspect-manifest.golden

@@ -0,0 +1,16 @@
+{
+	"schemaVersion": 2,
+	"mediaType": "application/vnd.docker.distribution.manifest.v2+json",
+	"config": {
+		"mediaType": "application/vnd.docker.container.image.v1+json",
+		"size": 1520,
+		"digest": "sha256:7328f6f8b41890597575cbaadc884e7386ae0acc53b747401ebce5cf0d624560"
+	},
+	"layers": [
+		{
+			"mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
+			"size": 1990402,
+			"digest": "sha256:88286f41530e93dffd4b964e1db22ce4939fffa4a4c665dab8591fbab03d4926"
+		}
+	]
+}