add support for optional layers on dirs->proxy fallback

[shakyShane]

In this example, if a request for / is not found on disk in examples/kitchen-sink, then the request is forwarded to the proxy.

At that point, the options for the route are lost, this doesn't work as you'd expect it to

servers:
  - bind_address: 0.0.0.0:3000
    routes:
      - path: /
        dir: examples/kitchen-sink
      - path: /
        proxy: https://browsersync.io
        delay:
          ms: 2000 # This won't take effect