Continue to serve assets when an absolute URL is passed to servePath.

Incorporates and replaces #258, fixes #257.

- `options.servePath` continues to operate in the same way it does today while paths are being generated. 
- If a fully-qualified URL is passed to `options.servePath` (like "http://localhost:3000/assets") assets used to be no longer served (because the requested URL would never be a fully-qualified URL). 
- Now, assets will be served at the pathname of the fully-qualified URL if a fully-qualified URL is passed to `options.servePath`.

As I played with the code a bit, it seemed to make most sense to add a property to the `options` object for the pathname of the servePath.
I don’t love the name of this, and am open to better ideas.