TargetBlankLinkProcessor.php

<?php

class TargetBlankLinkProcessor implements DocumentProcessorInterface
{
    /**
     * @param Document $document
     *
     * @return void
     */
    public function processDocument(Document $document)
    {
        $walker = $document->walker();
        while ($event = $walker->next()) {
            $node = $event->getNode();

            // Only stop at Link nodes when we first encounter them
            if (!($node instanceof Link) || !$event->isEntering()) {
                continue;
            }

            $node->data['attributes']['target'] = '_blank';
        }
    }
}