(http-url-arg http-url key) -> string or #f
Returns the value associated with the given key in the "args" field of the given <http-url>.
Example:
>> (define a (string->url
"http://en.wikipedia.org/wiki/Special:Search?search=Lisp&fulltext=fulltext&sourceid=mozilla-search"))
:: #f >> (http-url-args a) :: [dict ("fulltext" . "fulltext") ("search" . "Lisp") ("sourceid" . "mozilla-search")] >> (http-url-arg a "search") :: "Lisp"