Allow to send report to stderr

[umherirrender]

PHPUnit has a config to send all text to stderr

documentation of the XML Configuration File

phpunit-slow-test-detector does not check this config and send always to stdout (via echo).
The text from phpunit-slow-test-detector is sometimes inside the progress bar in this case (depending when stderr or stdout get flushed).

It is possible to check the config and send to stderr instead? Thanks.