Add an option to allow copying image indexes alone

The new --multi-arch option allows the user to select between copying the
image associated with the system platform, all images in the index, or
just the index itself without attempting to copy the images.

Signed-off-by: James Hewitt <james.hewitt@uk.ibm.com>

completions/bash/skopeo

@@ -40,6 +40,7 @@ _skopeo_copy() {
     --src-authfile
     --dest-authfile
     --format -f
+    --multi-arch
     --sign-by
     --src-creds --screds
     --src-cert-dir