Expose the "app_versions" of the version message to application code

[exarkun]

Currently the version message (or maybe "the version phase message"?) is handled but no attempt is made to parse anything from app_versions or expose it to application code.

Some applications may want to use the payload for feature/compatibility negotiation. The app :: Session -> IO a accepted by MagicWormhole.runClient should gain access to the app_versions payload somehow.