Allow defroute to bind params to a symbol.

I've been using secretary (very happily I might add) and I've come upon a scenario a few times where I think it would be useful to bind the url params directly to a symbol as one can do in compojure. 

Eg. I often find myself doing 

```clj
(defroute "/user/:id" [id] (do-something user-page {:id id}))
``` 

where it would be more natural to just do

```clj
(defroute "/user/:id" params (do-something user-page params))
```

Of course the real benefit of the second form is that if your page functions deal with the url params themselves, you can express the routing logic very succinctly as something like:

```clj
(route-mapping
  "/user/:id" user-page
  "/search"  search-page
  ...)
```