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/escape.go

@@ -0,0 +1,8 @@
+package progress
+
+const (
+	escape = "\x1b"
+	reset  = escape + "[0m"
+	red    = escape + "[31m"
+	green  = escape + "[32m"
+)