Introduce network priorities.

In mac os (probably also the other OSes), it is possible to specify the order/priority of network interfaces. It would be good if network-interfaces would also give insight on this.

<img width="518" alt="Screen Shot 2019-04-18 at 10 49 14" src="https://user-images.githubusercontent.com/914122/56331679-4de94f00-61c8-11e9-85db-3b3b9064c2b2.png">


