add jpeg generation, refactor

README.md

@@ -16,4 +16,4 @@ This will generate an image in current directory.
 
 See **./barnsley-fern --help** for available flags.
 
-Image on this page was generated with **./barnsley-fern-go -h 500 -v 500 -rainbow**
+Image on this page was generated with **./barnsley-fern-go -x 500 -y 500 -c 3 -out example/barnsley-fern.png**

image.go

@@ -35,7 +35,7 @@ func createBarnsleyFernImage(s *settings, w io.Writer) error {
 		}
 
 	default:
-		return fmt.Errorf("%w: %d", s.OM)
+		return fmt.Errorf("%w: %d", errUnknownFormat, s.OM)
 	}
 
 	return nil