Support overridable options

[clubby789]

It would be useful to have a way to specify an option which uses its last occurrence as its value; e.g.

cmd --foo fizz --foo buzz

assert_eq!(matches.opt_str("foo"), Some("buzz"))

This is useful for tools which are passed automatically generated arguments which a user may wish to override by appending their own (motivated by rust-lang/rust#133072)