added check for img visibility

Here's a sketch in jQuery for checking visibility.  

Use case:  The image is in a div that is toggled (ie click to show image).  
```
<div style="display: none">
  <img class="lazy" data-src="some-image.jpg" />
</div>

<script>
var lazy = new Riloadr(
  name: 'lazyload',
  defer: {mode: 'invisible'},
  breakpoints: [{name: 'all', minWidth: 0}]
);

$('div').click(function (e) {
  $(this).toggle();
  lazy.riload();
}
</script>
```