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/ChildDef/Optional.php

@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * Definition that allows a set of elements, and allows no children.
+ * @note This is a hack to reuse code from HTMLPurifier_ChildDef_Required,
+ *       really, one shouldn't inherit from the other.  Only altered behavior
+ *       is to overload a returned false with an array.  Thus, it will never
+ *       return false.
+ */
+class HTMLPurifier_ChildDef_Optional extends HTMLPurifier_ChildDef_Required
+{
+    public $allow_empty = true;
+    public $type = 'optional';
+    public function validateChildren($tokens_of_children, $config, $context) {
+        $result = parent::validateChildren($tokens_of_children, $config, $context);
+        // we assume that $tokens_of_children is not modified
+        if ($result === false) {
+            if (empty($tokens_of_children)) return true;
+            elseif ($this->whitespace) return $tokens_of_children;
+            else return array();
+        }
+        return $result;
+    }
+}
+
+// vim: et sw=4 sts=4