Track lines to prevent inconsistency on buffer reload from disk

[yuriescl]

Currently, if the file is modified on disk and :e is used to reload the buffer, the bookmark locations on that file will become incorrect if lines were added/removed from the file externally (not using the same vim instance).
This also happens with vim built-in marks.

There could be a more sophisticated tracking of the bookmarks to prevent losing them in case there's an external change to the file and the buffer is reloaded. Maybe save the line content when :w is used or something like that.