Better servePath behaviour when it's an absolute path and not a CDN (helpful in staging environments)

When servePath is an absolute path (starts with `//` or `http://`), `connect-assets` will not serve those assets, but will rely on them being available on another server like a CDN. This makes it complicated in some dev/test/staging environments (like mine) to do something like `servePath: "http://localhost:3000/baseURL/assets"`, because those assets will not be served and the browser will throw a 404.

This pull request adds a few lines of code that allows URLs like the one above to serve assets locally. To allow the old behaviour (when deploying to a CDN, for instance), I added a `cdn` options flag (defaults to false).