Expose `http.ServeMux` to allow applications to add additional endpoints

[benpye]

It would be useful to expose the http.ServeMux so an application could add an additional listener without needing to listen on a new port. In my case I want to expose a Prometheus metric endpoint, currently I need to listen on an additional point but I don't think there is any reason this should be necessary.

[brutella]

I've refactored hc and made a new library out of it. The new library is available as hap.
The new hap server gives you access to the ServeMux.