add 'movePointsWithLine' option to addMovableLineSegment

Summary:
specifying movePointsWithLine: true will cause movablePoints specifying
a line segment to move with the segment when the segment is dragged.

Test Plan: Tested locally in perseus.

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4738