Add an escape hatch to register an arbitrary route handler

[jbearer]

We should have something like api.raw(route_name, handler) where handler: (RequestParams, &State) -> Response, allowing the handler to build a response arbitrarily. This could be a simple wrapper around tide::ResponseBuilder. Example use cases:

• a route that always returns binary data (Vec<u8>) without having to go through the extra step of bincode serializing that data
• a Prometheus route
• non-RESTful routes in general