Broken background-image paths

[cyruscollier]

Our asset build structure has images in a separate directory from css, so it's css/main.css and images/*. Because of that, our CSS background images all have relative urls ../images/*. The generated styleguide.css path is hardcoded inside appRoot with no way to control the path, so it assumed that all images are within the same directory for every project. I suppose I could use a gulp filter to rewrite image paths on the CSS, but it seems like would be cleaner to provide a config option like stylesheetPath that could be used for all the generated stylesheets.