A start at allowing custom hubs in Wolverine.
Tested using the WolverineChat sample project.

Basically do opts.UseSignalR<MyCustomHub> and app.MapWolverineSignalRHub<MyCustomHub>()
where MyCustomHub inherits from WolverineHub. The old extensions for UseSignalR and MapWolverineSignalRHub remains and uses the default WolverineHub

I made WolverineHub.ReceiveMessage virtual to allow customization as well.
(And hopefully people remember to call base.ReceiveMessage as well...)

There's still no support for a hub that uses [Authorize] or has other ways of authenticating.
I was planning on looking into that later.