HTML5 <menu> element support.

[rafalwrzeszcz]

Added support for <menu> element for Menu module.

[ericf]

@tilomitra this makes sense to me, do you want to incorporate <menu> element support into your Menus revamp work?

[jamesalley]

This is a good PR, but I'm going to close it because it's no longer relevant given the new Menu, which uses low-specificity selectors based on classnames instead of <ul> or <menu> elements.

In the not-too-distant future, I may drop support for Menu from Pure (breaking it out into a new component library, perhaps); the styling of a menu is just too varied and too opinionated to belong in Pure.