fix(url_for): initialize relative_url function

Author: curbengh

lib/plugins/helper/url_for.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const url = require('url');
+const relative_url = require('./relative_url');
 
 function urlForHelper(path = '/', options) {
   if (path[0] === '#' || path.startsWith('//')) {
@@ -22,7 +23,7 @@ function urlForHelper(path = '/', options) {
 
   // Resolve relative url
   if (options.relative) {
-    return this.relative_url(this.path, path);
+    return relative_url(this.path, path);
   }
 
   // Prepend root path