title: Append element to Eslasticsearch field
body: <p>please is possible to append some element to elasticsearch field if types doesn't match? If I have document like this:</p>
  
  <pre><code>{
  "counter" : 1,
  "tags" : "red"
  }
  </code></pre>
  
  <p>and i want to append another tag field f.e "blue" like this:</p>
  
  <pre><code>{
  "script" : {
      "source": "ctx._source.counter += params.newTag",
      "lang": "painless",
      "params" : {
          "newTag" : "blue"
      }
  }
  }
  </code></pre>
  
  <p>and i want to have result similar to:</p>
  
  <pre><code>{
  "counter" : 1,
  "tags" : ["red", "blue"]
   }
  </code></pre>
  
  <p>i know this: </p>
  
  <pre><code>"source": "ctx._source.counter += params.newTag"
  </code></pre>
  
  <p>is used for append string to another string</p>
  
  <p>and this:</p>
  
  <pre><code>"source": "ctx._source.counter.add(params.newTag)"
  </code></pre>
  
  <p>for appending another element to list. So is there any way how to append another element to string field ? Thank You for any suggestions.</p>