Support for <picture> based alternative file formats

[jsonn]

It would be nice to support alternative image formats, e.g. webp for modern browers as alternative to jpeg. I'm willing to look into the implementation, but it would be nice to agree on the format first.

Option 1: additional options to specify alternative formats.
Option 2: allow multiple white-space separated URLs in the data-at-$width fields and compute the file type based on the extension.

I think the second option is easier to use and would be my preference.