[WIP] Sitemap generation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #132 
| License       | MIT
| Doc PR        | symfony/symfony-docs#123https://github.com/symfony-cmf/symfony-cmf-docs/issues/607

Started working on that feature. That would be the task:

- [x] implement generator for phpcr content
- [x] create decider for content which should be published
- [x] create controller for the output
- [x] general configuration and optional activating of the phpcr generator
no, it's an internal parameter and as such, it is prefixed with an underscore. The only option is typehinting for a Request instance and doing `$request->attributes->get('_format')`. But I don't like that, what's wrong with this? :)