Send message from web containers to the parent app

Currently web containers are a one way thing. You cannot call a Jasonette action from inside web container, for example.

Making this two-way could be helpful.

The issue names no files, tests, or entry points to begin from. First determine whether this request concerns documentation or a product change, then clarify how web containers should invoke Jasonette actions and what behavior is expected; done would mean that agreed two-way messaging behavior is specified and verified.

Written by the indexing model from the issue text.