Add uri callback to counterparty:// scheme

[mandelmonkey]

I am using opening freewallet from my mobile game using
counterparty:?message=MSG&action=sign&callback=URL

This works as expected, freewallet prompts the user to ok the signing and sends the results to the URL I specified.
It would be nice if freewallet could then redirect back to the mobile app

this could be done by adding another parameter like
counterparty:?message=MSG&action=sign&callback=URL&redirect=somegameuri