Add more flexibility to addMovableAngle

Summary:
 * Adds support for coordinate pairs for addMovableAngle points
 * Adds a fixed option to addMovableAngle, which disables the default
   move handling

Test Plan: * Used in the Transformer Widget in Perseus

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5534