Upgrade url_for to search for resources relative to their destination path

...as well as their source paths. This would fix #818.

I'm not 100% sure I like this (it feels like we're trying too hard, and I'm worried we'd "find" links that we shouldn't, or that people would lose what little concept they have of how Middleman is supposed to work) but it would fix some confusion we've seen.

Take special note of the change I had to make to the directory_indexes test.