Add HTMLPurifier to extlib

HTMLPurifier defangs arbitrary submitted HTML. We're using it in the
OStatus plugin, but it may be valuable for other parts of the codebase
(I think OEmbed might benefit, for example).

extlib/HTMLPurifier/HTMLPurifier.func.php

@@ -0,0 +1,23 @@
+<?php
+
+/**
+ * @file
+ * Defines a function wrapper for HTML Purifier for quick use.
+ * @note ''HTMLPurifier()'' is NOT the same as ''new HTMLPurifier()''
+ */
+
+/**
+ * Purify HTML.
+ * @param $html String HTML to purify
+ * @param $config Configuration to use, can be any value accepted by
+ *        HTMLPurifier_Config::create()
+ */
+function HTMLPurifier($html, $config = null) {
+    static $purifier = false;
+    if (!$purifier) {
+        $purifier = new HTMLPurifier();
+    }
+    return $purifier->purify($html, $config);
+}
+
+// vim: et sw=4 sts=4