feat_req: allow custom `--help` when `help` feature is disabled

[ver-nyan]

Would it be possible to remove the restriction for allowing custom --help flag when the help feature is disabled?

I rly like the derive style and minimal bin size of argh. There's a specific way that I want my help message to be, and also having an empty description leaves an extra new line in the message, so in the end I'm rolling with my own const HELP: &str = ... that I'd like to dump since it'll be easier for me to customize/maintain anyways.