Responsive image support

[johnnovak]

glightbox itself supports the data-srcset and data-sizes attributes for responsive images, could you please add support for that?

Check the last example here: https://github.com/biati-digital/glightbox/blob/master/README.md

[kinostl]

@johnnovak I was able to use this technique to get responsive images. It requires attr_list

![alt-text](/images/file.png){srcset="/thumbs/file.png 400w"}