Support select().observable in mockDOMSource

It's now possible to mock select('.foo').observable using mockDOMSource:

const userEvents = mockDOMSource({
  '.foo': {
    'click': Rx.Observable.just({target: {}}),
    'mouseover': Rx.Observable.just({target: {}}),
    observable: Rx.Observable.empty() // <---- this
  },
  '.bar': {
    'scroll': Rx.Observable.just({target: {}})
  }
});

Read more: https://github.com/cyclejs/cycle-dom/issues/106