Associate a handler from the application with each route

[pictographer]

This is the key missing bit of functionality to make tide-disco usable.

routefinder::Router::add(...) takes a route specification and a generic handler. Specify a way for the application to define the handlers and give the handlers access to the application state as well as any needed accessors for the web server state, e.g. healthcheck status.

[jbearer]

This was done in #21