title: btn-xs no longer a valid option in bootstrap 4?
body: <p>I just migrated to bootstrap 4 and my xs buttons no longer seem to be extra small!</p>
  
  <p>Looking at the <a href="http://v4-alpha.getbootstrap.com/components/buttons/" rel="noreferrer">docs for buttons in bootstrap 4</a> I don't see the option for extra small buttons?</p>
  
  <p>Can anyone confirm this for me?</p>
  
  <p>Thanks!</p>