Add support for reading params as URL-encoded JSON from the URL fragment

Using iframe.name to pass data works well on the web, but not from apps. They use WebViews instead of iframes, and it's hard or not possible to define the name variable synchronously from the app-code. URLs are easier to debug both for apps and web, but using query-parameters are limiting if we want a “scope” for each plugin to pass parameters to the ext frame. So using a JSON in the URL is the best of both, but IE has a max URL-length of 2083 characters, so on the web we should fallback to using iframe.name if the URL-length is too long.