Added the functions mc/mark-next-lines and mc/mark-previous-lines
which create cursors one line above and below point.
Refactored common mc/mark-previous-like-this and
mc/mark-next-like-this functionality into mc/mark-more-like-this.
Changed mc/mark-next-like-this and mc/mark-more-like-this to call
mc/mark-next-lines and mc/mark-previous-lines when there is no
active region (instead of erroring).
