Add kpod diff command

kpod diff reports on differences between two layers, specified as
layer IDs, containers, or images.  In the case of containers or
images, kpod diff produces a diff for the top layer

Signed-off-by: Ryan Cole <rcyoalne@gmail.com>

cmd/kpod/main.go

@@ -22,6 +22,7 @@ func main() {
 	app.Version = Version
 
 	app.Commands = []cli.Command{
+		diffCommand,
 		exportCommand,
 		historyCommand,
 		imagesCommand,