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

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.

No file or test is named in the issue. Start by locating the restriction that rejects a custom `--help` when the `help` feature is disabled, then trace the derive-style configuration around it. Done means a custom help message is accepted without enabling the `help` feature, including when `description` is empty.

Written by the indexing model from the issue text.