F# API - extended spawn functions for supervisor strategy definition

I've noticed that current `spawn` function doesn't allow programmers to define a custom supervisor strategies. I've extended them by defining `spawns` function, which takes one more parameter - SupervisorStrategy builder function. 

Example usage:
```

let oneforone (system:ActorSystem) name (decider: Exception -> Directive) (f: Actor<'m> -> Cont<'m,'v>) =
    spawns system name (fun() -> OneForOneStrategy(System.Func<_,_>(decider)) :> SupervisorStrategy) f

let decider (e:Exception) =
    match e with
    | :? ArgumentNullException as exc -> Directive.Escalate
    | _ -> Directive.Restart 

let act = 
    oneforone system "one" decider 
    <| fun mailbox -> ...
```

I'm not sure if presented `oneforone` (an analogously `allforone`) should be a part of F# API. @thinkbeforecoding could you review that one?