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 :)