Raw methods
They are the reason why structopt is so flexible.
Each and every method from clap::App and clap::Arg can be used directly -
just #[structopt(method_name = single_arg)] or #[structopt(method_name(arg1, arg2))]
and it just works. As long as method_name is not one of the magical methods -
it's just a method call.
Note: Raw methods are direct replacement for pre-0.3 structopt's
#[structopt(raw(...))] attributes, any time you would have used a raw() attribute
in 0.2 you should use raw method in 0.3.
Unfortunately, old raw attributes collide with clap::Arg::raw method. To explicitly
warn users of this change we allow #[structopt(raw())] only with true or false
literals (this method is supposed to be called only with true anyway).