Ability to override json encoder and decoder arguments

I wanted to serialize objects using ```for_json``` methods, but I found it cumbersome to pass the ```for_json=True``` argument to simplejson when using Flask's ```jsonify``` method. Since there are other useful overrides during (de)serialization, I propose that Flask's config-object should be able to override all json options. The already supported ascii, sort_keys, prettyprint and indent-options are still supported, and take precedence over any new values.