Add "print-config" subcommand (#698)

* Add "show-config" subcommand

* Add integration tests

* Handle wrong additional args

* Support "BackupFilenameExpand" option

* Rename to print-config

* Remove config nil check as it will never be nil

* Rework print config to work with config.resolve()

* Use defer unset()

* Add warning about feature not being stable yet

cmd/backup/main.go

@@ -11,8 +11,18 @@ func main() {
 	foreground := flag.Bool("foreground", false, "run the tool in the foreground")
 	profile := flag.String("profile", "", "collect runtime metrics and log them periodically on the given cron expression")
 	flag.Parse()
-
+	additionalArgs := flag.Args()
 	c := newCommand()
+
+	if len(additionalArgs) > 0 {
+		switch additionalArgs[0] {
+		case "print-config":
+			c.must(runPrintConfig())
+			return
+		default:
+			panic("unknown command: " + additionalArgs[0])
+		}
+	}
 	if *foreground {
 		opts := foregroundOpts{
 			profileCronExpression: *profile,