title: FullCalendar custom button icon not showing
body: <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>      $('#calendar').fullCalendar({
           header: {
                      left: /*'prev,next'*/'BackwardButton,ForwardButton',
                      center: 'title',
                      right: '',
                  },             
                  customButtons: {                    
                      ForwardButton: {
                          //text: 'Framåt',
                          click: function () {
                          
                          }
                      },
                      BackwardButton: {
                          //text: 'Bakåt',
                          click: function () {
                           
                              
  
  
  
                          }
                      }
                  },
                                  buttonsIcons: {
                      BackwardButton: 'left-single-arrow',
                      ForwardButton: 'right-single-arrow',
                  },
        
        });</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;link href="https://cdnjs.cloudflare.com/ajax/libs/fullcalendar/3.9.0/fullcalendar.css" rel="stylesheet"/&gt;
  
  &lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.22.0/moment.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/fullcalendar/3.9.0/fullcalendar.js"&gt;&lt;/script&gt;
  
  
  
  &lt;div id="calendar"&gt;&lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I am working with <a href="https://fullcalendar.io" rel="noreferrer">FullCalender.io</a>. I have two custom buttons that I would like to add the default chevron or arrow to. I found these <a href="https://fullcalendar.io/docs/buttonIcons" rel="noreferrer">properties</a>, but I can't figure out where in the full calendar initialization I should put it. Nothing have worked so far. I only get the text 'undefined'.  Any ides anybody?</p>
  
  <pre><code>      jQuery('#calendar').fullCalendar({
              header: {
                  left: 'BackwardButton,ForwardButton',
                  center: 'title',
                  right: '',
              },             
              customButtons: {                    
                  ForwardButton: {
                      //text: 'Framåt',
                      click: function () {
  
                      }
                  },
                  BackwardButton: {
                      //text: 'Bakåt',
                      click: function () {
  
                  }
              },
              buttonsIcons: {
                  BackwardButton: 'left-single-arrow',
                  ForwardButton: 'right-single-arrow',
              }
          });
  </code></pre>