CommandOptions now has an IsHidden property that, when set to true, will cause the option to be hidden from the following cases:
- Help text using `-h|--help`
- Xml representations generated with the `cli xml` command
- Diagnostics displayed with the `cli explain` command
Hidden options can still be outputted with `cli explain` using the `--hidden` option that is also used to display hidden commands.
Fixes#631