Added json response mode for implicit flow

Hi,

Thanks for the great product.

Would it make sense to add a json response mode to implicit flow? Or will that open some security vulnerabilities?

I am thinking about the following scenario:
1. Implicit flow with javascript client (SPA) is used. No consent screens are used.
2. The user is authenticated through a cookie, however SPA client does not have an access token yet (or maybe it expired).
3. The client performs an AJAX request to the protected resource.

At this point I would like to fetch a new access token using AJAX transparently for the user (without redirecting to the login page). 

In this case I can:
1. Catch the 401 error in AJAX handler
2. Submit the authorization request, obtain a new access token (the user is still authenticated).
3. Repeat the original request with a new access token.

In order to achieve that I would need to return the access token in a json format.

What do you think about this scenario? If it makes sense, I can also add some unit tests for this case.

Thanks,
Dennis.