data-src

Love the gem, thanks!

Using the helper method, my images are rendering src instead of ```data-src```. Is there a way the ```src``` attribute be ```data-src```?

Example returns
```
<%= cl_image_tag(item.photo.key, crop: "fill", flags: "lossy", fetch_format: "auto", width: 640, quality: "auto", client_hits: "true", sizes: "100vw") %>
```

// alt image url
```

```