Expose HelpMessage and allow for custom help switches

[msrd0]

This pull request introduces a HelpMessage trait that is implemented for every struct that derives FromArgs, allowing the user to manually display the help message generated by argh. Also, the user can specify #[argh(disable_help = true)] which disables the built-in help flag and allows the user to add custom help flags, allowing for shorthands like -h or -? and similar customizations. Closes #30

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[msrd0]

@googlebot I signed it!

[jyn514]

I would be very interested in having this for https://github.com/poliorcetics/cargo-intraconv :)

[erickt]

Other than that, it looks good to me. Could you rebase, and make sure you format with cargo fmt?

[erickt]

Stylistically, I think flipping this to "enable_help" reads a bit better. That avoids devs having to mentally parse expressions like if !disable_help { ... }.

[msrd0]

I chose disable_help because as a dev I usually assume optional boolean flags to default to false, but I can change that if you want

[erickt]

Why an additional trait instead of adding a method to FromArgs?

[msrd0]

The help message only exists for structs, not for enums, so it's not implemented for every type that implements FromArgs