Better image optimisation pipeline

[patricoferris]

Sesame has basic support for image responsiveness, but actually currently it is a little broken:

1. I don't think the produced img and srcset are correct, I think the way forward is to use the picture tag instead. This should be a relatively simple fix.
2. Part of way websites today either don't optimise their images or do so poorly is because of the initial up-front costs of doing so. Sesame should provide a more comprehensive default behaviour i.e. take images and an optimisation strategy and handle all of the path mangling behind the scenes. The user shouldn't have to decided where to put things. I think this makes sense and could put the images in public/static/<hash>/<image_name> and then re-write references in the markdown or whatever.