add ability to add multiple classes

[jmarca]

Would be nice to be able to add multiple classes, separated by spaces, as with setting classes in HTML

The REAMDE implies you can: "classname: {String} Additional class(es) to menu element container",

But you can't---no docs as to how, no docs in component/classes saying how, and no tests in either place to demonstrate that it can or cannot be done.

It seems like this line:

https://github.com/component/dropdown/blob/master/index.js#L47

is the way classes are set, so perhaps this feature request should be pushed upstream to component/classes??

I think a useful approach would be to allow adding multiple classes separated by whitespace, as with HTML, and the way d3 does it.