Allow more context in IURLModifer.Modify

Hi,

We are trying to serve the Cassette MSBUILD precompiled files from a static CDN but Cassette always adds Cassette.axd / filename into a string.

The code to decouple the URL and form the new CDN url is never neat.

In UrlGenerator - The CreateBundleURL method should pass the bundle reference into Modify URL of IURLGenerator or some sort of context so we can decide on which URL we should generate.

Thoughts?