Added expression provider

This PR add a Symfony Expression Language provier.

This provides a very dynamic way to genereate URLs, for example:

````
stdClass:
    uri_schema: /{path}
    token_providers:
        path: [expression, { expression: 'parameter.cmf_routing.basepath + "/" slugify(subject.title) }]
````

The expression language is not modified within the provider and it is up to the developer to provide a suitably featureful language to the provider. For example one which provides access to container parameters, a slugifier and the locale.

We could provide either a fully featured ExpressionLanguage in this component, or provide one which is composable somehow.