cmd/dist: implement fetch prototype

With the rename of fetch to fetch-object, we now introduce the `fetch`
command. It will fetch all of the resources required for an image into
the content store. We'll still need to follow this up with metadata
registration but this is a good start.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

progress/doc.go

@@ -0,0 +1,2 @@
+// Package progress assists in displaying human readable progress information.
+package progress