title: Elasticsearch alias type to reference multi fields
body: <p>I'm using Elasticsearch alias fields to rename some fields in my mapping without needing to reindex everything. Works great. Now I want to rename some multi fields as well, like this:</p>
  
  <pre><code>"message":{
    "type":"text",
    "fields":{
      "raw":{
        "type":"keyword"
      }
    }
  }
  </code></pre>
  
  <p>That's an analyzed field named <code>message</code> and a keyword version named <code>message.raw</code>. I can make an alias for <code>message</code> like this:</p>
  
  <pre><code>"title":{
    "type":"alias",
    "path":"message"
  }
  </code></pre>
  
  <p>But I cannot seem to figure out how to alias the <code>message.raw</code> field. I tried different things.</p>
  
  <ol>
  <li><p>Suspected that this would work out of the box by just using title.raw and hoping that Elasticsearch would convert it to message.raw. Didn't work.</p></li>
  <li><p>Specified <code>raw</code> as a field in <code>title</code>:</p></li>
  </ol>
  
  <pre><code>"title":{
      "type":"alias",
      "path":"message",
      "fields":{
          "raw":{
              "type":"alias",
              "path":"message.raw"
          }
      }
  }
  </code></pre>
  
  <p>This generates the following error when updating the mapping: Mapping definition for [title] has unsupported parameters: [fields : {raw={type=alias, path=message.raw}}]</p>
  
  <ol start="3">
  <li>Specified <code>title.raw</code> as a new field:</li>
  </ol>
  
  <pre><code>"title":{
      "type":"alias",
      "path":"message"
  },
  "title.raw":{
      "type":"alias",
      "path":"message.raw"
  }
  </code></pre>
  
  <p>This results in an error: Cannot merge a field alias mapping [title] with a mapping that is not for a field alias.</p>
  
  <p>Is this supported somehow?</p>