Add a dropdown prop to Button which toggles a Dropdown when the button is clicked

[nkohari]

Depends on #702.

We should add a dropdown prop to Button. When present, clicking the button will toggle the display of the specified Dropdown.

Here's an example API:

const dropdown = (
  <Dropdown>
    <Dropdown.Option ...>...</Dropdown.Option>
    ...
  </Dropdown>
)

return <Button dropdown={dropdown} />