window.onresize to trigger individual images update

What would be the best way for individual images to subscribe to `resize` events without keeping reference to these elements?

Probably by subscribing to the `onImagesReplaced` on `Imager.ready`.

```js
var Imager = require('imager.jsx');
var ResponsiveImage = Imager({ onResize: true });

React.createClass({
  render: function(){
    return (
      <main>
        <header><ResponsiveImage src="..." alt="Site banner" /></header>

        <ResponsiveImage src="..." />
      </main>
    );
  }
});
```