Picture Element Lazy Loading

Firstly, thanks fo creating this amazing tool. I use it on almost every website I build. I would, however, like to open a discussion about the way that picture elements are lazy-loaded.

Why not just put the class `lozad` on the picture element and add then, when it enters the viewport, change all data-srcset attributes to srcset attributes on the source elements and on the img element, do the same but also change data-src elements to src elements.

The markup would then just become

```

```

Adding the `data-iesrc` attributes to the picture elements etc just seems confusing to me and I'm not sure what benefit it brings.

I would be very happy to submit a PR if this is something you are interested in?