Remove url_for helper reference in img tag

lib/plugins/tag/img.js

@@ -1,5 +1,6 @@
 'use strict';
 
+var url = require('url');
 var util = require('hexo-util');
 var htmlTag = util.htmlTag;
 
@@ -13,7 +14,26 @@ var rMeta = /["']?([^"']+)?["']?\s*["']?([^"']+)?["']?/;
 *   {% img [class names] /path/to/image [width] [height] [title text [alt text]] %}
 */
 module.exports = function(ctx) {
+  var config = ctx.config;
+
+  function makeUrl(path) {
+    if (path[0] === '#' || path.substring(0, 2) === '//') {
+      return path;
+    }
+
+    var data = url.parse(path);
+
+    if (data && data.protocol) {
+      return path;
+    }
+
+    path = config.root + path;
+
+    return path.replace(/\/{2,}/g, '/');
+  }
+
   return function imgTag(args, content) {
+
     var classes = [];
     var meta = '';
     var width;
@@ -24,18 +44,17 @@ module.exports = function(ctx) {
     var item = '';
     var i = 0;
     var len = args.length;
-    var url_for = ctx.extend.helper.get('url_for');
 
     // Find image URL and class name
     for (; i < len; i++) {
       item = args[i];
 
       if (rUrl.test(item)) {
-        src = url_for.call(ctx, item);
+        src = makeUrl(item);
         break;
       } else {
         if (item[0] === '/') {
-          src = url_for.call(ctx, item);
+          src = makeUrl(item);
           break;
         } else {
           classes.push(item);