Add edit command

Various refactoring:
- reduced repetition with conversation message handling
- made some functions internal

main.go

@@ -9,7 +9,7 @@ import (
 
 func main() {
 	if err := cli.Execute(); err != nil {
-		fmt.Fprint(os.Stderr, err)
+		fmt.Fprintln(os.Stderr, err.Error())
 		os.Exit(1)
 	}
 }