Initial arrowheads-at-line-and-ray-ends functionality

Summary: Arrows are clipped at the edge at some angles. :(

Test Plan: http://exercises.ka.local/exercises/graphing_linear_equations.html

Reviewers: joel

Reviewed By: joel

CC: alpert

Maniphest Tasks: T1973

Differential Revision: http://phabricator.khanacademy.org/D5637