Create a standard server feature interface for listing & configuring server prefixes and/or ports

[Tratcher]

Depends on #329

Once we have this then Hosting can use it to Console.WriteLine each address after IServerFactory.Start. See #328

[Tratcher]

Recomendation:

public interface IServerAddressesFeature
{
  IList<string> Addresses { get; }
}

[davidfowl]

Feels like we should break it into parts

[Tratcher]

We had a long discussion on that. The server may support a variety of formats, so only it can really break it into meaningful parts. e.g. http vs unix sockets vs named pipes, etc..

Note Add may have some normalization, like "8080" turning into "http://*:8080/". Add may also throw if you give it unrecognized formats.

[davidfowl]

We also need to support the --port flag. Should that be another issue?

[Tratcher]

We had the idea today to directly support ports in server.uls. If you just provided an int then we would assume you meant http://*:port/ (or localhost?). The same goes for the IServerAddressesFeature.