title: Bootstrap 3: adding a new set of columns
body: <p>I've been using <code>Bootstrap 3</code> for a while and now I need to make a new set of extra small columns for horizontal mobiles (e.g. <code>384px</code> screen width) and after this use it as follows:</p>
  
  <p><code>col-xxs-1</code>, <code>col-xxs-2</code>, <code>col-xxs-offset-5</code>, <code>hidden-xxs</code>, etc.</p>
  
  <p>Are there some <code>Bootstrap</code> <code>Less</code> mixins for this purpose? I'm not sure how to use them</p>
  
  <p>edit:</p>
  
  <p>There is a <code>Bootstrap</code> mixin called <code>.make-grid()</code>, but I can't make it work.</p>