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.
offset이 넘었을 때, 마지막 노드를 선택하도록 하는 방어코드 같네요.