Add support mutating args once added

[kbknapp]

It'd be nice to be able to mutate aspects of certain args after they've been add to an App struct.

What I'd like to be able to do is add a bunch of args via the App::args_from_usage then later go back and mutate the two or three outlyers that require some special setting, like possible_values or something.

[joshtriplett]

One possible interface to this would be a new App method mod_arg<F: FnMut(Arg) -> Arg>(name: &str, f: F):

let app = App::new("cmd").args_from_usage("...");
app.mod_arg("somearg", |arg| arg.possible_values(...));

[kbknapp]

Has been implemented on v3-master