feat(rxTokenInterceptor): Allow exclusions for interceptor token

Closes #557

By default, don't send the X-Auth-Token to rackcdn.com requests. This
fixes the problem of browsers sending pre-flight OPTIONS requests
to the CDN when grabbing the navigation JSON file.

`rxTokenInterceptor` is now a provider, so the list of exclusion
domains can be customized at config time, via
`TokenInterceptor.exclusionList`