Router: matching query string support.

The "query" option matches decoded arguments, including plus ('+') to
space (' ').  Like "uri", it can be a string or an array of strings.

src/nxt_http.h

@@ -148,6 +148,7 @@ struct nxt_http_request_s {
     nxt_str_t                       *path;
     nxt_str_t                       *args;
 
+    nxt_str_t                       args_decoded;
     nxt_array_t                     *arguments;  /* of nxt_http_name_value_t */
     nxt_array_t                     *cookies;    /* of nxt_http_name_value_t */
     nxt_list_t                      *fields;