Decouple elements of `api`

[daveliepmann]

This issue is created per my Clojurians Slack convo with @ikitommi on 15 February 2017.

When introducing compojure-api to existing compojure projects, one often prefers à la carte access to the functionality provided by api (e.g. the Swagger UI, existing routes, and middleware).

For example:

(root (api {:swagger {:data {:info {:title "api1"}}}} ...)
      (api {:swagger {:data {:info {:title "api2"}}}} ...)
      my-existing-routes
      (resources “/” ...))`