resolves #201 make PostCSS url filter Windows-compatible

The regex passed to the PostCSS url filter is checked against the relative path between the current working directory and the asset location. Under Windows, this path contains backslashes instead of slashes.

Instead of making the regex compatible with Windows paths (which sadly involves quadruple backslashes), we pass a predicate function to the filter property and check the same regex against the asset's pathname, which contains forward slashes both on Windows and Unix-like systems.