Feature/history in a line

I reduced dual undo stack into one with stackOffset.
This patch also fixes wrong bookmark implementation.

And changed recording undo stack timing from 'before modifying text' to 'after modifying text'. The first undo stack is recorded while initiating summernote.
With `list.initial`, `dom.fromOffsetPath` tests are entirely failed. These tests haven't been failed with wrong test sets - There was no `class="note-editable"` on sample dom.

Thus, I think list.initial is not necessary or we have to fix it on `fromOffsetPath`.