New: API methods rows().nodes(), rows().data() and columns().header()

- rows().nodes() - Get the TR nodes for the selected rows
- rows().data() - Get the data for the selected rows
- columns().header() - Get the header cells for the selected columns

- This also introduces the `iterator` method to the API instance, which
  can be used to loop over selected columns / rows, for the context
  (tables), returning a flat array / API instance.