Making it possible to specify custom code and always adding using bhl directive to generated code

src/bind.inc.php

@@ -43,6 +43,8 @@ function codegen(
     $options['register_class'] = 'BHL_AutoBindings';
   if(!isset($options['namespace']))
     $options['namespace'] = 'bhl';
+  if(!isset($options['custom_code']))
+    $options['custom_code'] = '';
 
   if(isset($options['plugins']))
   {

tpl/codegen_autobind.twig

@@ -3,12 +3,15 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using bhl;
 #if !BHL_FRONT
 {%- for imp in imports ~%}
 using {{imp}};
 {%- endfor ~%}
 #endif
 
+{{custom_code}}
+
 {% import "bhl_bind_macro.twig" as macro %}
 
 namespace {{namespace}} {