Add a transform editorProps->widgetProps

Summary:
Adds a function that can be registed by the widget as a transformation
from its editor props to its renderer props.

Test Plan:
 * Make an input-number
 * try grading various correct/incorrect things
 * Make a plotter
 * Check that you don't get errors for widgets that don't have a
   transform specified (i.e. the plotter)

Reviewers: brianmerlob, justinjaffray

Reviewed By: justinjaffray

Subscribers: alex

Differential Revision: http://phabricator.khanacademy.org/D8372