Add support for img with srcset

[tyssen]

It would be good if the module could support both img with srcset, and picture markup. picture should only really be used when art direction comes into play; when just serving different sized images, img with srcset is recommended: http://blog.cloudfour.com/dont-use-picture-most-of-the-time/

[johannesdachsel]

I will definitely look into that. I'm not quite sure the art direction use case would work with this particular approach because the module just replaces a single image. But other than that, I think for just serving resized versions of that one image should be easily possible.