Summary: This diff adds the `client.onUnhandledMessage` handler, to be able to handle messages for which we don't know the names upfront. Some existing plugins that only send one kind of message can benefit from this, as can generic plugins, like the createTablePlugin
Reviewed By: passy
Differential Revision: D24949453
fbshipit-source-id: 0fed81e28aee350632c09ee3bb834f306dc8b100
33e527f39a