Include configured Slick paths when resolving via `image-url()`/`font-url()`

This change includes the configured loader and font paths when using the `image-url` and `font-url` helpers.

The use case here would be when Slick assets are stored within a particular directory _underneath_ whatever base paths the helper functions will prepend. Anyone who desires the previous behavior can just set `$slick-loader-path` and `$slick-font-path` to empty strings.