Add keyboard support to dropdown.js

**For peer review only: Looking for feedback**

This PR adds proper keyboard support to dropdown.js, based on keyboard interaction rules from WAI-ARIA Authoring Practices for [Menu](http://www.w3.org/TR/wai-aria-practices/#menu)

Some highlights:
* `Enter`, `Space`, and `Up`/ `Down` Arrow keys will display menu, placing focus on first menu item
* `Up`/ `Down` arrow keys cycle focus through items
* `Tab`, `Esc`, and `Left` Arrow keys dismiss menu, placing focus back on trigger
* Letter keys [a-z] cycle focus through items that begin with that letter

Also updated are all dropdown examples to encourage good a11y habits.

Additions:
* Add `role="separator"` for divider items
* Add `role="heading"` for header items
* Add `aria-disabled="true"` to disabled items

Once this has been reviewed, I can add proper unit tests.