Make a more generic enabledFeatures object

Summary:
Previously, we passed enableHighlight around everywhere.
Instead, we should pass an object of which features to enable, so that
as we add more such features, we won't clutter things up.

Test Plan:
 * check widget highlighting on test
 * substate and check that there are no js errors, and use the
   react console to check that enableHighlight is defined for
   WidgetContainers
 * click the score button on testrender.html, and make sure the
   input-number highlights.

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D8591