title: Spring Data Elasticsearch - Create keyword field with normalizer
body: <p>We are using the spring-data-elasticsearch project to interface with our elasticsearch clusters, and have been using it now for around a year. Recently, we moved to elasticsearch 5.x (from 2.x) where we now have the "keyword" datatype.</p>
  
  <p>I would like to index these keywords as lowercase values, which I know can be done with field normalizers. I can't find anywhere in the documentation or online where I can add a normalizer to a field through the annotation based mapping. 
  E.g</p>
  
  <pre><code>@Field(type = FieldType.keyword, &lt;some_other_param = some_normalizer&gt;)
  </code></pre>
  
  <p>Is this something that can be done? I know that we can use JSON based mapping definitions as well, so I will fall back to that option if needed, but would like to be able to do it this way if possible.</p>
  
  <p>Any help would be very appreciated!</p>