Support deleting an image tag

If the Docker Registry API allows for images to be deleted, issuing the
command `delete <image> <tag>` will result in the tag being removed from
the server.

A limitation of this change is that it will cause any unreferenced tags
to become orphaned, requiring the garbage collector within the docker
registry server to clean these up.  This is because there is no way to
efficiently determine if each of the layers are reused by other tags or
images on the server.

Implement delete logic

Fix broken unit tests

README.md

@@ -32,6 +32,7 @@ Options:
   -V, --version
           Print version
 ```
+
 ### Checking the API Version
 
 Perform a simple API Version check towards the registry endpoint
@@ -87,6 +88,15 @@ Options:
 
 Delete a tagged image from the registry
 
+Note! This requires that the registry has storage delete rights enabled. For
+example, when creating the registry, setting the environment variable
+`REGISTRY_STORAGE_DELETE_ENABLED=true` to enable that feature. If that is not
+enabled, a `MethodNotAllowed` error will be returned.
+
+Note! This will only remove the tag from the registry, it will not remove
+orphaned digests. For that, the garbage collector on the registry service must
+be run separately.
+
 ```shell
 Usage: dredge <REGISTRY> delete <IMAGE> <TAG>