Add fallthrough support for Kubernetes (#626)

* Add fallthrough support for Kubernetes

This enables registering other services in the same zone as
Kubernetes services. This also re-orders the middleware chain
so that Kubernetes comes before other types, in order to make
this work out-of-the-box.

* Remove extra line

middleware/kubernetes/handler.go

@@ -71,6 +71,9 @@ func (k Kubernetes) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.M
 		_, _, err = middleware.A(&k, zone, state, nil, middleware.Options{})
 	}
 	if k.IsNameError(err) {
+		if k.Fallthrough {
+			return middleware.NextOrFailure(k.Name(), k.Next, ctx, w, r)
+		}
 		// Make err nil when returning here, so we don't log spam for NXDOMAIN.
 		return middleware.BackendError(&k, zone, dns.RcodeNameError, state, nil /*debug*/, nil /* err */, middleware.Options{})
 	}