Add the Admin extension that enables editing HTML attributes

Contains an interface that encapsulates the attributes related methods for the menuNode.
Relies on burgov/key-value-form-bundle for the admin.
I wasn't sure about where should I put it in the config tree. I think it makes sense to put it after use_sonata_admin since it's admin related. The default config dump will look like this after moving:
```
cmf_menu:
    persistence:
        phpcr:
            enabled:              false
            menu_basepath:        /cms/menu
            content_basepath:     /cms/content
            prefetch:             10
            manager_name:         ~
            menu_document_class:  Symfony\Cmf\Bundle\MenuBundle\Doctrine\Phpcr\Menu
            node_document_class:  Symfony\Cmf\Bundle\MenuBundle\Doctrine\Phpcr\MenuNode
            use_sonata_admin:     auto
            edit_attributes:
                enabled:              false
            menu_admin_class:     Symfony\Cmf\Bundle\MenuBundle\Admin\MenuAdmin
            node_admin_class:     Symfony\Cmf\Bundle\MenuBundle\Admin\MenuNodeAdmin
            admin_recursive_breadcrumbs:  true
    content_url_generator:  router
    allow_empty_items:    false
    voters:
        content_identity:
            content_key:          ~
        uri_prefix:           false
    publish_workflow:
        enabled:              true

```