Keyword only args

[aaltat]

Fixes #9

[pekkaklarck]

I only looked the code and it looks fine except that it totally ignores kwonly args without defaults. An annoying thing with handling defaults in general is that the getfullargspec returns normal args and their possible defaults differently than kwonly args and their possible defaults. On Robot side this is handled so that ArgumentSpec has positional and kwonlyargs which both contain only argument names (i.e. are lists) and then separate defaults that is a mapping from argument name (either normal or kwonly) to their default value. Something like that, including similar ArgumentSpec object, could work here as well. Let's talk more on Slack!

I also added some comments related how the code possibly could be simplified.