Add general mapping options for a type

This is an implementation based on this [issue](https://github.com/FriendsOfSymfony/FOSElasticaBundle/issues/340). This pull-request allows the user to add a `mapping_options` field in the config file to customize a mapping for each type.
This allows us to support [_ttl](http://www.elasticsearch.org/guide/reference/mapping/ttl-field/), [_timestamp](http://www.elasticsearch.org/guide/reference/mapping/timestamp-field/), [_size](http://www.elasticsearch.org/guide/reference/mapping/size-field/) and other general mapping options for each type.