Added the option to show protractor in Interactive Graph widget.

Test Plan:
Toggled the protractor option while editing various subwidgets and observed
no interference with other options. Verified that the option properly gets
exported to JSON, and that it then renders as expected.

Reviewers: jack, eater

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4485