Add kwargs support for memoize

Happy to update docs 

TODO: check performance impact of using `inspect.getcallargs` (it on its own does fallbacks depending how signature looks like)

@Suor should I add feature proposal Issue on GH ?
I updated the signature of `ip_to_city`, and `request_city_from_slow_service` to illustrate function that can return up to `max` most accurate cities for given ip...

@Suor do you recon I should:
- keep old name
- update request_city_from_slow_service -> request_cities_from_slow_service
- come up with other example