Supply initial_state attribute for jsinput tag

[Sorry, repo associated with last pull request with same name got reverted before merge, resulting in no changes.  Hopefully this time for sure...]

Supplying an initial value for the stored state of a jsinput widget allows user to provide a start-up configuration for the external tool, which will then get updated on subsequent saves/checks as they interact with the tool. Very useful for a generic tool (eg, a schematic editor) that we'd like to configure differently for different instances (eg, components in parts bin, available simulation modules, initial schematic).

Goal: allow course to supply a stateful interaction (with grading) that doesn't require the heavy lifting of running an XBlock through the submission and approval process.
@terman Instead of ```undefined``` I would put here the value of the yaml template, like for the other optional attributes: ```'{"selectedObjects":{"cube":true,"cylinder":false}}'```