ADD: lazy load images for theme Next: #1649 PART 4

This add compare fir `lazy` classname and if it true, will add 'data-original' attrubute to replace it with `src` attrubute, to make image load lazy.
If no `lazy` class defined, image tag will work at standart state.

lib/plugins/tag/img.js

@@ -40,6 +40,7 @@ module.exports = function(ctx) {
     var height;
     var title;
     var alt;
+    var lazysrc = '';
     var src;
     var item = '';
     var i = 0;
@@ -85,7 +86,14 @@ module.exports = function(ctx) {
       alt = match[2];
     }
 
+    // Find lazy load classname
+	  if (classes[0] === 'lazy') {
+  		lazysrc = src;
+  		src = '/images/loading.gif';
+  	}
+
     var attrs = {
+      'data-original': lazysrc,
       src: src,
       class: classes.join(' '),
       width: width,