Add diff service implementation

Add snapshot subcommand to ctr for creating diffs of RW layers.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

cmd/ctr/main.go

@@ -61,6 +61,7 @@ containerd client
 		execCommand,
 		pauseCommand,
 		resumeCommand,
+		snapshotCommand,
 		versionCommand,
 	}
 	app.Commands = append(app.Commands, extraCmds...)