ExceptionHandler::attachListener(callable $listener) : void added

[vaclavvanik]

As in Zend\Stratigility\Middleware\ErrorHandler I added attachListener method to ExceptionHandler. Listeners are triggered when an exception is caught and provide ability to log exception.

Attach listeners using ExceptionHandler::attachListener():

$exceptionHandler->attachListener(function (Throwable $throwable) use ($logger) {
    $message = sprintf(
        '[%s] %s',
        date('Y-m-d H:i:s'),
        $throwable->getMessage()
    );
    $logger->error($message);
});

Tests are added but I don't know how to test triggering listners because exception handler calls php exit. :-/