Placeholder for subviews uses the same tag as the subviews root tag.

Space-pen's subview method uses a div as a placeholder for subviews to be inserted on postProssesing.
This has a bug on tables, as discovered on https://github.com/atom/space-pen/issues/31, because `<div>` is not a valid child of `<tbody>` and ends up rendered outside the table before replacing it with the subview.
I propose to use the same tag as the subviews root-tag as a placeholder, this way it won't be misplaced before postProssesing.
By the way, loving this library so far!!!