use buffered output in `list` command

https://github.com/ouch-org/ouch/blob/b2d0ff75c89a1dd2d60938d0a490b320efcc1a43/src/list.rs#L35

All output to this file should be done with `BufWriter`.

(On top of that, `writeln!` shouldn't be mixed with `print!`, we should only use the former.)