Pass parameters through FB app_data object

I found myself needing to pass parameters through `fb_dispatch` and there was no way to do this with the current version of carrotbook.  Since it looks like carrotbook is getting deprecated, I figure I'd throw this here, in case it's useful. (Since there is no `renderer.rb` it may not be applicable, you can make the call)

an example of how this middleware extension works:

`render fb_dispatch: "?sk=app_#{Carrotbook.configuration.fb_app_id}&app_data={\"path\":\"#{finalists_path}\",\"params\":{\"show_card\":\"3\",\"foo\":\"bar\"}}"`

By adding the **params** object within the `app_data` object, we can now set Rack's `env['QUERY_STRING']` and pass parameters around within facebook.