Added macro param-push, simplifying a lot of code

I've added a new macro, param-push, that simplifies some of the code in cl-reddit.lisp

The macro could be extended to work for some more extensive syntax, though I've yet to do this. 

It is currently undocumented but here's an example:

(macroexpand-1 '(param-push
     after before count limit show target))

=>

(PROGN
 (WHEN TARGET (PUSH `("target" . TARGET) PARAMS))
 (WHEN SHOW (PUSH `("show" . SHOW) PARAMS))
 (WHEN LIMIT (PUSH `("limit" . LIMIT) PARAMS))
 (WHEN COUNT (PUSH `("count" . COUNT) PARAMS))
 (WHEN BEFORE (PUSH `("before" . BEFORE) PARAMS))
 (WHEN AFTER (PUSH `("after" . AFTER) PARAMS)))