Pass model in highlight helper

Pass model to callback of highlight helper for consistency with layout helper.

lib/helpers/highlight.js

@@ -15,7 +15,7 @@ helpers.highlight = function(lang, cb){
 
   // cb() is simply a user-defined function. It could (and should) contain
   // buffer additions, so we call it...
-  cb();
+  cb( this.model );
 
   // ... and then use fromMark() to grab the output added by cb().
   var cbOutLines = this.buffer.fromMark(startMark);
@@ -35,4 +35,4 @@ helpers.highlight = function(lang, cb){
 
   // returning is allowed, but could cause surprising effects. A return
   // value will be directly added to the output directly following the above.
-}
+}