[TASK] Add support for ES multi_fields

Add a support for multi_fields mapping support. See the official documentation:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/_multi_fields.html

The configuration can be done with the normal Mapping annotation, like this::

    @ElasticSearch\Mapping(index_name="name", analyzer="custom_french_analyzer", multi_fields={@ElasticSearch\Mapping(index_name="nameHasTag", analyzer="tag_analyzer")})

The root mapping define the first field in the list, and all the mapping in the multi_fields attributes define the other mapping. No changes are needed for the indexing.