Feature Page component

This adds support for a `Page` entity, which should be a minimal CMS implementation.

Currently a single `RouteLoader` is available and should be defined in your `routing.yml` as follows:
```yml
my_cms_route_name:
    resource: .
    type: elcodi_page_render
```

Everything else is made configurable by the `elcodi_page` key.
```yml
elcodi_page:
    mapping: ~ # usual mapping setup
    routing:
        loader:      elcodi.core.page.router.simple_loader.loader
        route_name:  elcodi_page_render_view
        route_path: '/{path}'
        controller:  elcodi.core.page.controller.page:renderAction
```

TO DO:
- [ ] add `replace` entries in `composer.json`.
- [ ] ease route configuration.
- [ ] multiple implementations of `RouteLoader`.
- [ ] gather feedback for changes.
- [ ] adapt to [fix/abstract-extension](https://github.com/xphere-forks/elcodi/compare/fix/abstract-extension) if/when it is accepted.
This is no necessary, we're not working with Repository interfaces. If we want to do that, we should change all the repositories and the castings in our services.

Is that what we want?