Previously using mc/mark-lines in a fowards/backwards/forwards
combination would cause multiple cursors to be placed at the same
point of the same line. This is not useful behaviour.
Noted while implementing magnars suggestion: https://github.com/magnars/multiple-cursors.el/pull/23#commitcomment-1983183
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).
