title: how to select row wise maximum and tag it by column name
body: <p>I have a data frame of 5 columns with probability values. </p>
  
  <p>The 5 columns are the target variable values I want one additional column that tags the maximum probability for each row with the target value. help? </p>
  
  <p>For example:</p>
  
  <pre><code>  id    columnA columnB columnC columnD FinalTag 
    1112  0.653   0.33    0.01    0.006    "A"
  </code></pre>