Render the icon stack.
        
        For example:
        <span class="icon-stack">
          <i class="icon-check-empty icon-stack-base"></i>
          <i class="icon-twitter"></i>
        </span>
        <span class="icon-stack">
          <i class="icon-circle icon-stack-base"></i>
          <i class="icon-flag icon-light"></i>
        </span>
        <span class="icon-stack">
          <i class="icon-sign-blank icon-stack-base"></i>
          <i class="icon-terminal icon-light"></i>
        </span>
        <span class="icon-stack">
          <i class="icon-camera"></i>
          <i class="icon-ban-circle icon-stack-base text-error"></i>
        </span>

def render(self):
        """
        Render the icon stack.
        
        For example:
        <span class="icon-stack">
          <i class="icon-check-empty icon-stack-base"></i>
          <i class="icon-twitter"></i>
        </span>
        <span class="icon-stack">
          <i class="icon-circle icon-stack-base"></i>
          <i class="icon-flag icon-light"></i>
        </span>
        <span class="icon-stack">
          <i class="icon-sign-blank icon-stack-base"></i>
          <i class="icon-terminal icon-light"></i>
        </span>
        <span class="icon-stack">
          <i class="icon-camera"></i>
          <i class="icon-ban-circle icon-stack-base text-error"></i>
        </span>
        """
        return '<span class="icon-stack">%s</span>' % (
            ''.join([item.render(['icon-stack-base'] if i == 0 else []) for (i, item) in enumerate(self.items)])
        )