in jsown:filter allow to filter by list of keys

Allows to use lists as parameters to jsown:filter. Useful for when you need to filter several keys from deep within target json.

```lisp
(defparameter *data* (flexi-streams:octets-to-string (drakma:http-request "http://www.reddit.com/r/programming/.json")))
(setf *json* (jsown:parse *data*))
(jsown:filter *json* "data" "children" map "data" '("title" "url")) ;;returns list of pairs (title url) of current frontpage stories
```