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.
I agree with your point.
However, this code works well in webkit-based or Firefox browsers. Then how about to hold this approach before applying new way to bookmark? (like tagging as you mentioned)