backup: use datetime string as default archive name

Because unix timestamp is not very human readable.

cmd/backup.go

@@ -33,7 +33,7 @@ portable among all supported database engines.`,
 		boolFlag("verbose, v", "Show process details"),
 		stringFlag("tempdir, t", os.TempDir(), "Temporary directory path"),
 		stringFlag("target", "./", "Target directory path to save backup archive"),
-		stringFlag("archive-name", fmt.Sprintf("gogs-backup-%d.zip", time.Now().Unix()), "Name of backup archive"),
+		stringFlag("archive-name", fmt.Sprintf("gogs-backup-%s.zip", time.Now().Format("20060102150405")), "Name of backup archive"),
 		boolFlag("database-only", "Only dump database"),
 		boolFlag("exclude-repos", "Exclude repositories"),
 	},