Auto add --version to print program version

[Boscop]

Please automatically add --version to print program version: env!("CARGO_PKG_VERSION").
Also I think it'd make sense if --help prints the version as well.

[benbrittain]

This library is currently used most widely through build systems that are not Cargo based. I don't think that cargo package version is always guaranteed to match the version surfaced to users either. It's pretty trivial to add this flag yourself, so for now I'd like to hold off on this feature request.

[suluke]

FWIW I didn't find it that trivial to add a --version flag in case one has (otherwise) required options. But I came up with a solution here: https://gist.github.com/suluke/e0c672492126be0a4f3b4f0e1115d77c
Just replace argh::from_env with this module's argh_cargo::from_env. Since it more or less just wraps argh::from_env internally the integration should be clean-ish. The way the help text is extended is obviously a bit naive, but so is the current help text formatting in general IIUIC looking at https://github.com/google/argh/blob/master/argh_shared/src/lib.rs#L18. If this is something anyone would like to see upstreamed I'd be happy to create a PR.

[homeworkprod]

I agree that it's not that trivial.

Given a program that requires positional arguments, one could not simply add a --version switch and just print the version string because myprogram --version is aborted early with "Required positional arguments not provided: […]`.

Regarding CARGO_PKG_VERSION: How about defaulting to its value while providing an optional way to override it?

[SamuelMarks]

Thanks @suluke that's useful. Would be great to see this integrated into the core library, with @homeworkprod's suggestion of CARGO_PKG_VERSION being just the default.