Allow custom NotFound handler

I would like to use a custom handler for unmatched requests, so we can track metrics and return a custom error message. This is currently not supported, since hostrouter always invokes `http.Error`:

https://github.com/go-chi/hostrouter/blob/03da900f6d45aaf0d5a1f392e2bafe8437ae5102/hostrouter.go#L44

It would be very nice if we could set a custom handler for that case, similar to [chi's NotFound hook](https://go-chi.io/#/pages/routing?id=making-custom-404-and-405-handlers)

Start at hostrouter.go line 44, where unmatched requests currently invoke http.Error, and compare the requested behavior with chi's NotFound hook. Define how a custom handler should be configured for unmatched requests, then verify that it can track metrics and return a custom error message.

Written by the indexing model from the issue text.