relative assumes input file path === output file path

[kilianc]

when using this in an asset pipeline it often happens that you bundle everything in your css folder into bundle.min.css and in my case the output file is in the root.

assuming I have css/main.css it resolves correctly images/foo.png into ../images/foo.png but this breaks the output bundle.

I would suggest to allow a string to be passed to relative representing the path you want to resolve against.

[borodean]

@kilianc 4.1.0 is now capable of receiving a string as relative option value. If a string is provided, generated paths would be relative to a path described in it.