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.
Chrome / Safari does not have default selected range after initialization. But Firefox gives whole `<body>` as selected, so I workaround-ed here. I'll add comment here, thanks!