title: Bootstrap add focus and validation states to input-group-addon, not just input
body: <p>In Bootstrap, when you :focus on an input it adds a blue border and box shadow to indicate the focus.</p>
  
  <p>For validation states (error, warning, success), it adds a red, yellow, and green border to the input respectively.</p>
  
  <p>However, if you have placed an input-group-addon to the input field, the addon does not focus.  Creating a somewhat weird effect:</p>
  
  <p><img src="https://i.stack.imgur.com/wiyks.png" alt=":focus state"></p>
  
  <p><img src="https://i.stack.imgur.com/mSVAi.png" alt=".has-error state"></p>
  
  <p>How can I add the focus to the addon?</p>