Overwriting css in-place fails silently

With the command `cssbeautify-cli -f style.css -w style.css` I expect the file to beautify in-place but this it not what happens. It ends up with a one-line file:

```css
/**** style.css ***/
```

Obviously this is not what I want. I think that cssbeautify-cli should either support this behavior, or give a warning/prevent users from doing this.