Ability to resolve :pre and :handler values at request-time

* An item in :pre can be anything - the value can be resolved at request time with the addition of a lookup-map: (middleware/wrap-match my-routes my-pre-mapping)
* :handler can be an ifn or a symbol - the value can be resolved at request time with the addition of a lookup-map: (middleware/wrap-match my-routes my-pre-mapping my-handler-mapping)