title: Elasticsearch mapping, date field with null_value
body: <p>Is it possible to have a mapping in Elasticsearch with null_value that equals to "now" in a field with type "date"? Something like this:</p>
  
  <pre><code>"mappings": {
    "doc": {
      "properties": {
        "last_updated": {
          "type": "date",
          "format": "yyyy-MM-dd HH:mm:ss",
          "null_value": "now"
        }
      }
    }
  }
  </code></pre>
  
  <p>So when I search the last_updated field will be equal to the current time, if no value exists?</p>