Relative paths for images/fonts.

Stylesheets that are imported inside the main stylesheet get relative paths. However if the path/url starts with https, http, ftp or data it stays the same.

## Example

#### main.css
@import "../js/foo.css";

#### foo.css:
.bg { background: url('../img/bg.png');



#### Result:
.bg { background: url('../js/../img/bg.png');