Add HTML.Noopener to add a noopener rel to every external link

This has performance benefits https://jakearchibald.com/2016/performance-benefits-of-rel-noopener/ but most importantly also security benefits https://mathiasbynens.github.io/rel-noopener/

Adresses https://github.com/ezyang/htmlpurifier/issues/96

library/HTMLPurifier/HTMLModule/Noopener.php

@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * Module adds the noopener attribute transformation to a tags.  It
+ * is enabled by HTML.Noopener
+ */
+class HTMLPurifier_HTMLModule_Noopener extends HTMLPurifier_HTMLModule
+{
+
+    /**
+     * @type string
+     */
+    public $name = 'Noopener';
+
+    /**
+     * @param HTMLPurifier_Config $config
+     */
+    public function setup($config)
+    {
+        $a = $this->addBlankElement('a');
+        $a->attr_transform_post[] = new HTMLPurifier_AttrTransform_Noopener();
+    }
+}
+
+// vim: et sw=4 sts=4