Fix support for additional parameters support in flex mixins
Fix support for additional parameters support in flex mixins

cf. #31

girassolbit/kouto-swiss,leny/kouto-swiss,greyhwndz/kouto-swiss,girassolbit/kouto-swiss,krkn/kouto-swiss,greyhwndz/kouto-swiss,leny/kouto-swiss,krkn/kouto-swiss