Allow easy selection of output interface (stdin/stderr)

[mortenscheel]

Scenario: You have a command that outputs raw data (like json or csv) to the console, and it uses laravel/prompts. You want to save the output to a file, so you run php artisan some:command > path/to/file.json.
Unfortunately the prompt is using stdout, so it gets sent to the file and you don't see anything in the console.

I figured out a workaround that makes the prompt use stderr:

$output = $this->getOutput()->getOutput();
if ($output instanceof ConsoleOutputInterface) {
    Prompt::setOutput($output->getErrorOutput());
}

It works great, but it would be nice if there was an easier and more intuitive way of doing it. Maybe an optional argument that allows you to pass an OutputInterface, or a static Prompt::stderr() method. And maybe consider using stderr by default in the next major release, unless there are any drawbacks that I'm not aware of.

[driesvints]

Heya, if you could send in a PR we could look at we can consider it maybe. Thanks for your suggestion.