[+FEATURE] FLOW3 (MVC): Content Negotiation support in ActionController
This adds support for content negotiation based on the Accept HTTP header.
If a format other than "html" is accepted by the client and the format is
listed in a controller's "supportedFormats" array, this format is chosen.
If a format has been explicitly defined through a @format property of the
current route, that format is used instead.
In any other case "html" is used for web requests and "txt" for command line