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 ?
Oh, I didn't realize you complicated the example. I think it better just be left as is.

Highlighting that kwargs will also work is not really needed, people will just assume it does. Or did it confuse you first time?