Matcher based configuration

With this PR, it is now possible to configure how comments renderer behaves when rendering comments for a content.

## Behavior
* By default, if no configuration is present, comments are open for every content.
* Possibility to enabled/disable comments by configuration.
* Possibility to define a specific provider for a given content.
* Uses eZ Publish built-in matcher factory, used in kernel for selecting content/location/block views. It's of course possible to combine matchers.

## Configuration example
```yaml
ez_comments:
    system:
        ezdemo_frontend_group:
            default_provider: disqus
            content_comments:
                # Here we want to disable comments specifically for Content #138.
                articles_disabled:
                    match:
                        Id\Content: [138]
                    enabled: false
                # We enable comments for article and blog_post ContentTypes
                articles:
                    match:
                        Identifier\ContentType: [article, blog_post]
                    enabled: true
                # We want to use facebook comments in a specific section
                facebook_comments:
                    match:
                        Identifier\Section: [social_section]
                    provider: facebook
                    # Note that comments are enabled by default if not specified.
```