Write messages to stderr instead of stdout

[tzakharko]

I am writing an utility that works as a filter (it produces output on stdout that is then processed further), so any diagnostic messages (e.g. command line help) must show up on stderr instead. For example, if --help is used, I want the help message to be printed to stdout and the tool exiting with a non-zero exit code. I didn't find anything in documentation, but maybe I am just looking in a wrong place.

[tzakharko]

For now I have solved it with

App::new(...).
...
 .get_matches_safe()
 .unwrap_or_else(|err| {
     eprintln!("{}", err.message);
     std::process::exit(1);
});

It works, but feels a bit ugly to me. Anyway, it would be nice if Error:: use_stderr() were configurable in some way instead of hard-coded like it is now.

[pksunkara]

Could you create an issue please about the configuration? Thanks