Expose Editor in the API

[define-null]

This commit makes it possible to use Editor directly. My main motivation was to provide the possibility to access ExternalPrinter. This is mainly
needed for scenarios where additional threads are spawned from the repl and start printing output. In case those threads do not use external printer the output is mixed with the input that the user might be writing. On the other hand, if external printer is used, the console is updates and if the use has written something already, this line remains intact.

Example of how this is used:

    let mut rl = ClapEditor::<ReplCommand>::new_with_prompt(">> ");
    let mut printer =
        rl.get_editor().create_external_printer().unwrap();

    let (tx, rx) = std::sync::mpsc::channel();
    thread::spawn(move|| {
        for received in rx {
            printer.print(received).unwrap()
        }
    });
   ...