Add Router.withNotFound to handle unknown URL

[Tarmil]

Fixes #308.

Example use:

type Page =
    | [<EndPoint "/">] Home
    | [<EndPoint "/not-found">] NotFound

type Model =
    { page: Page }

type Message =
    | PageChanged of Page

let router =
    Router.infer PageChanged (fun m -> m.page)
    |> Router.withNotFound NotFound

This will send the message PageChanged NotFound

• during initialization (before first display) if the URL couldn't be parsed;
• after the URL was programmatically set to a value that can't be parsed.

It does not change the behavior when the user clicks a link to a URL that can't be parsed: these are considered external links, and standard navigation is performed.

Breaking changes

• The interface IRouter<'model, 'msg> has an additional property NotFound: 'msg option.
• The record Router<'endpoint, 'model, 'msg> has
  • additional field makeMessage: 'endpoint -> 'msg;
  • additional field notFound: 'endpoint option;
  • type of field setRoute has changed to string -> 'endpoint option instead of string -> 'msg option.

These should be transparent for most users.