Make Asset Pipeline friendly

Right now Asset Pipeline helpers return values like `url("assets/foo/bar.png")` or `assets/foo/bar.png`. Either way it maintains the extension so mo ends up spitting out paths like `url("assets/foo/bar.png").png` and `assets/foo/bar.png@2x.png`, etc.

This fixes that. It's a little less friendly but will fit into more pipelines. Take it or leave it. :pizza: