implement callback function feature

README.md

@@ -21,6 +21,16 @@ With the HTML in place you can then initialize the plugin using the factory meth
 lazyload();
 ```
 
+If you prefer you can pass callback function. When the image is loaded, the function defined as 'onLoadCallback' is called.
+
+```js
+lazyload(null, {
+    onLoadCallback: function() {
+        console.log('callback function is called!');
+    }
+});
+```
+
 If you prefer you can explicitly pass the image elements to the factory. Use this for example if you use different class name.
 
 ```js

lazyload.js

@@ -110,6 +110,9 @@
                             if (srcset) {
                                 entry.target.srcset = srcset;
                             }
+                            if(self.settings.onLoadCallback) {
+                                entry.target.onload = self.settings.onLoadCallback;
+                            }
                         } else {
                             entry.target.style.backgroundImage = "url(" + src + ")";
                         }