Don't depend on markup for lazy load the images

I was setting Slick up for a project which needed images lazy loading. I did just like the recommended markup said: added the `data-lazy` property. Unfortunately, I spent a nice portion of time trying to figure out why it was not working. When I read the source code I could understand what was happening:

The lazy load strategy depends on user's markup. The `src` tag could not be present in the HTML, because Slick would understand it as loaded. Now, it depends solely on `data-lazy` attribute: its presence will force the load, and its absence will indicate it is already loaded.

I think this may help people in future and also it makes more sense, since it is now markup independent.