fix syntax highlight by using tempfile.yaml over tempfile

cmd/pttodo-cli/cli.go

@@ -57,8 +57,8 @@ func edit(dry bool, filepath string) error {
 			g.Close()
 		}
 		f.Close()
-		tempFile = f.Name()
-		return nil
+		tempFile = f.Name() + ".yaml"
+		return os.Rename(f.Name(), tempFile)
 	}
 	vi := func() error {
 		bin := "vim"