Renderer widget props upgrading

Summary: Adds an upgrade step for widget props to the renderer

Test Plan:
yolo!
In testing, I added the following to input-number's widget
exports to make a fake silly upgrade that will run all the
time, which turns the answer type to "decimal", and can
be seen by looking at the acceptable formats shown
when the input-number is focused:

    version: {
        major: 1,
        minor: 0
    },
    propUpgrades: {
        1: (oldProps) => {
            var newProps =_.extend(oldProps, {
                answerType: "decimal"
            });
            console.log("upgrade!", oldProps, newProps);
            return newProps;
        }
    }

This seemed to work as I would expect

Reviewers: charlie, joel

Reviewed By: charlie, joel

Subscribers: alex

Differential Revision: http://phabricator.khanacademy.org/D10524