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/Definition.php

@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * Super-class for definition datatype objects, implements serialization
+ * functions for the class.
+ */
+abstract class HTMLPurifier_Definition
+{
+
+    /**
+     * Has setup() been called yet?
+     */
+    public $setup = false;
+
+    /**
+     * What type of definition is it?
+     */
+    public $type;
+
+    /**
+     * Sets up the definition object into the final form, something
+     * not done by the constructor
+     * @param $config HTMLPurifier_Config instance
+     */
+    abstract protected function doSetup($config);
+
+    /**
+     * Setup function that aborts if already setup
+     * @param $config HTMLPurifier_Config instance
+     */
+    public function setup($config) {
+        if ($this->setup) return;
+        $this->setup = true;
+        $this->doSetup($config);
+    }
+
+}
+
+// vim: et sw=4 sts=4