mixin for concating class names
use buildClassName to preserve passed in class names
use js-stylesheets for default css
shift responsibilities down to menuitems

no reason for the menu to need to hold so much state, push
responsibility down to the MenuOptions
add disabled prop for MenuOption

MenuOption will fire callback method if user selects a diabled option
fix css type for Menu__MenuOptions

clean up some default styles
clean up basic example
added on click outside to auto close