title: Map feature selection output to column name (ChiSqSelector)
body: <p>I am trying to select the most predictive features for a dataset...</p>
  
  <pre><code>label   features
  class1  [2, 5.7, 3]
  class1  [6, 3.2, 1]
  ...
  </code></pre>
  
  <p>...using <a href="https://spark.apache.org/docs/1.6.0/ml-features.html#chisqselector" rel="nofollow">ChiSqSelector</a> on Spark:</p>
  
  <pre><code>val selector = new ChiSqSelector()
    .setNumTopFeatures(10)
    .setFeaturesCol("features")
    .setLabelCol("label")
    .setOutputCol("selectedFeatures")
  
  val result = selector.fit(predictions).transform(predictions)
  display(result)
  </code></pre>
  
  <p>When I do this, the output is simply the ranked column <em>values</em>:</p>
  
  <pre><code>selectedFeatures
  [5.7, 3, 2]
  [3.2, 1, 6]
  ...
  </code></pre>
  
  <p>However, I would to instead get the ranked column names... what is the best way to achieve this?</p>