[FEATURE] Added request format

__Tasks:__

- [ ] gather feedback for my changes
- [ ] submit changes to the documentation
- [x] Extract request format (https://github.com/sulu-cmf/sulu/pull/230)
- [x] implement example (https://github.com/sulu-cmf/sulu-standard/pull/218)


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/212
| BC Breaks     | 
| Doc           | https://github.com/sulu-cmf/docs/commit/ff44bde14a9aa5067c659c69d767f52689a1230d
When I think about it again, what @dantleech said really makes sense... I just checked, you really get trouble when you call a page with .rss now, which has no for `{_format}` in the template. It outputs the HTML with the wrong Content-Type. If you do it Dan's way, you'll probably receive a 406, as it should be.