Allow a jsonp response's callback name to correspond to the request

* Add a config property (`dynamic_jsonp`) defaulting this behavior to false
When dynamic_jsonp is true, ignore callback param

That way, if a URL changes the callback param, it will be considered a
match and the callback payload will be rewritten as usual. But if a URL
changes a non-callback param, it will be considered a miss.
Add documentation for c.dynamic_jsonp_keys