title: How to format numbers similar to Stack Overflow reputation format
body: <p>I want to convert a number into a string representation with a format similar to Stack Overflow reputation display.</p>
  
  <p>e.g.</p>
  
  <ul>
  <li>999 == '999'</li>
  <li>1000 == '1,000'</li>
  <li>9999 == '9,999'</li>
  <li>10000 == '10k'</li>
  <li>10100 == '10.1k'</li>
  </ul>