Allow passing data-properties to menu items.

Usage:

menu: [
{ title: 'item a', data: { id: 123, 'some-other-property': 42, action:
... }
]

which adds
data-id="123" data-some-other-property="42"
to the a-element.
Merge pull request #29 from francisrath/master

Allow passing data-properties to menu items.