add <menu>

[jonschlinkert]

See the mozilla docs for menu. I'd be happy to do a pr (I can also add li to resolve #3)

[TooTallNate]

Is it really a block level element though? It's not coming up on the list.

[jonschlinkert]

Since it fills the same role as <ul> and it isn't listed as an inline element either, I figured it was a block level element.

I only wanted to add it for completeness for http://breakdance.io/, since I was using the mozilla docs HTML for test fixtures, and <menu> was used a couple of times. But since it's experimental I understand if you'd rather not add it.