Display a hint on how to open the help when entering a unknown switch

For example: if I enter the `cp` command with a wrong switch, it displays how to show the help:

```
$ cp -y
cp: invalid option -- 'y'
Try 'cp --help' for more information.
```

If you don't want to hard code this, one option would be to give the programmer the ability to specifies an 'unknown option handler'.
