When promisifying, store does not preserve disableNunjucks property

This was causing difficulties in a less-maintained renderer I was using, but I think this may also count as a bug upstream.

lib/extend/renderer.js

@@ -46,13 +46,18 @@ Renderer.prototype.register = function(name, output, fn, sync) {
   name = getExtname(name);
   output = getExtname(output);
 
+  var disableNunjucks = fn.disableNunjucks;
   if (sync) {
     this.storeSync[name] = fn;
     this.storeSync[name].output = output;
 
     this.store[name] = Promise.method(fn);
+    this.store[name].disableNunjucks = disableNunjucks;
   } else {
-    if (fn.length > 2) fn = Promise.promisify(fn);
+    if (fn.length > 2) {
+      fn = Promise.promisify(fn);
+      fn.disableNunjucks = disableNunjucks;
+    }
     this.store[name] = fn;
   }