Support for srcset and relative URL for assets

Hi there,

First of all, thank you very much for this plugin!

I'd like to ask. Is it possible to add `srcset` to an img tag?

Also, is it possible to output relative URLs for assets based on page/ post or document, so assets could be organised in subdirectories? Similar to this plugin https://github.com/samrayner/jekyll-asset-path-plugin (but it doesn't support collections).
