Add a `setInputValue` to the Perseus API

Summary:
calling setInputValue(widgetId, valueString) change the value of an input-number or expression widget.
hopefully useful for clients to write their own input things.

Still rough, but hoping it's a decent v0 :)

Test Plan:
Run the following in the command line:

    itemRenderer.setInputValue("input-number 1", "5")
    itemRenderer.setInputValue("answer-area", "5")
    itemRenderer.setInputValue("answer-input-number 1", "5")

with the appropriate widgets existing, then
verified the widgets values changed as expected

Reviewers: charlie

Reviewed By: charlie

Subscribers: alex, laura, mgp

Differential Revision: http://phabricator.khanacademy.org/D9431