Change signature of Router.Create to deserialize to option

[Jand42]

For creating routers for server/client shared use, this would be more useful than current Router.Create:

    let CreateRouter (ser: 'T -> list<string>) (des: list<string> -> option<'T>) =
        {
            Parse = fun path ->
                match des path.Segments with
                | Some ep ->
                    Seq.singleton ({ path with Segments = [] }, ep)
                | None ->
                    Seq.empty
            Write = fun value ->
                Some (Seq.singleton (Route.Segment(ser value)))
        } : Router<'T>

Current one is deserializing to 'T, not option<'T>, which is ok for fully client-side routing use when one of the EndPoint cases are representing a wrong URI, but not good for server-side use when even resource links would be hijacked by Sitelets routing if set up that way. Changing signature is a breaking change, but probably less confusing than having two different functions for the same basic goal, but one only working in a specific side case.