title: Convert column index into corresponding column letter
body: <p>I need to convert a Google Spreadsheet column index into its corresponding letter value, for example, given a spreadsheet:</p>
  
  <p><img src="https://i.stack.imgur.com/DyaPs.png" alt="enter image description here"></p>
  
  <p>I need to do this (this function obviously does not exist, it's an example):</p>
  
  <pre><code>getColumnLetterByIndex(4);  // this should return "D"
  getColumnLetterByIndex(1);  // this should return "A"
  getColumnLetterByIndex(6);  // this should return "F"
  </code></pre>
  
  <p>Now, I don't recall exactly if the index starts from <code>0</code> or from <code>1</code>, anyway the concept should be clear.</p>
  
  <p>I didn't find anything about this on gas documentation.. am I blind? Any idea?</p>
  
  <p>Thank you</p>