Add new qualifier, -comment, to more easily identify specific jobs.

This qualifier allows specific text to be associated with a backup
job to easily associate what repository (or other context) a
duplicacy job is running on behalf of.

duplicacy/duplicacy_main.go

@@ -1846,6 +1846,10 @@ func main() {
 			Usage:    "enable the profiling tool and listen on the specified address:port",
 			Argument: "<address:port>",
 		},
+		cli.StringFlag{
+			Name:	"comment",
+			Usage:	"value that is ignored by duplicity (useful for identifying process)",
+		},
 	}
 
 	app.HideVersion = true