[fix] retina doesn't work in asset pipeline

asset pipeline helpers like asset-path return values like `url("assets/foo/bar.png")`, the way retina-bg-img was working, it would return values like `url("assets/foo/bar.png").png` or `url("assets/foo/bar.png.png")`. this is less convenient but makes it work in more contexts. feel free to merge or not
[bump] 0.3.0