Implement %AutoFormat.RemoveEmpty, end to start ref, and injector rewind.

Injector rewind: Injectors can now use the method rewind() in order to move
the input index backwards, so that they can reprocess tokens (other injectors
are not affected by a rewind). This functionality was necessary to implement
nested node removals in %AutoFormat.RemoveEmpty.

End to start ref: To facilitate rewinding, HTMLPurifier_Token_End now
maintains a reference called $start to the starting token for their node.

%AutoFormat.RemoveEmpty removes empty nodes. Lots of people have requested
it, so here is a partially effective implementation. Because it is implemented
as an Injector, it's not possible for it to handle newly introduced empty
nodes by later validators, specifically auto-closing and child validation.
The Injector is only meant to be used on HTML-ish languages.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

library/HTMLPurifier/Injector.php

@@ -54,6 +54,30 @@ abstract class HTMLPurifier_Injector
      */
     public $needed = array();
     
+    /**
+     * Index of inputTokens to rewind to.
+     */
+    protected $rewind = false;
+    
+    /**
+     * Rewind to a spot to re-perform processing. This is useful if you
+     * deleted a node, and now need to see if this change affected any
+     * earlier nodes. Rewinding does not affect other injectors, and can
+     * result in infinite loops if not used carefully.
+     */
+    public function rewind($index) {
+        $this->rewind = $index;
+    }
+    
+    /**
+     * Retrieves rewind, and then unsets it.
+     */
+    public function getRewind() {
+        $r = $this->rewind;
+        $this->rewind = false;
+        return $r;
+    }
+    
     /**
      * Prepares the injector by giving it the config and context objects:
      * this allows references to important variables to be made within