Improve the Before and After hook syntax and capabilities

[thecodejunkie]

Now that we have added the new route syntax, we need to make sure that the Before and After hooks are brought up to par. I propose the following changes

• Perform an overview of the declaration syntax and make it align with the route syntax better. This means supporting various overloads for sync and async versions
• Make it possible for them to be part of the ordinary route processing flow, meaning they would be able to pass through the content negotiation system. Currently the content negotiation is performed in the DefaultRouteInvoker, but hook handling is done in the InvokePreRequestHook and InvokePostRequestHook of NancyEngine (they're called before/after the DefaultRouteInvoker is called).