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.
Well, in this case, `PageRepository` is coupled to `EntityRepository`, but just an implementation.
We're opening here a door to implement other `PageRepositoryInterface` fully decoupled.