Optional data-driven router

Nobody has claimed this yet.

The `compojure.api.routes/Route` abstraction enables any routing lib to be used. There could be a examples for different routing libs and there could be even a own (collect-the-best-parts-of-existing-implementations) router too. One could go fully with macros, go fully data-driven or mixin freely.

```clj
;; the data-driven core, no sweet macros
(require '[compojure.api.sour :refer :all])

;; to mixin data & macros
;; (require '[compojure.api.core :refer :all])

(def app
(api
{:swagger
{:ui "/api-docs"
:spec "/swagger.json"}}
;; Pedestal-style router here.
(routes
#{["/todo" :post (resource {:handler (constantly (ok))})]})))
```

would be trivial to implement.

Start by reading the compojure.api.routes/Route abstraction and the routing example in the issue. Compare the proposed data-driven core with the existing macro-based approach and review examples for other routing libraries. Done would require a settled router design and working examples, but the issue does not define acceptance criteria or named tests.

Written by the indexing model from the issue text.