Implement smart usage

[kbknapp]

Usage strings, if possible should be re-generated upon error for the current usage case. For example if you have a non-required by default argument which requires another argument, and the user fails to supply the third argument (which is required by proxy), the usage string is generic - yet the error message says a required argument is missing...which argument?

let m = App::new("app")
                .arg(Arg::from_usage("--reqs").requires("input"))
                .arg_from_usage("[input] 'some input'")
                .get_matches();

Run with app --reqs and the usage does not identify [input] as a requirement, yet states a generic "required argument missing"

Now this does happen in certain instances, but I want to implement it in all instances.

[kbknapp]

Closed with #90

[kbknapp]

Usage strings are now recalculated on errors, so that the current usage attempt is a template and all requirements/conflicts are spelled out in the usage string.