title: Call footerCallback by variable DataTable
body: <p>I create a Datatable and I add rows by method "row.add"</p>
  
  <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>var table = $('#example').DataTable();
  ...
  ...
  ...
  table.row.add(...)
  ...
  ...
  ...</code></pre>
  </div>
  </div>
  </p>
  
  <p>I want sum some columns by method "footerCallback"</p>
  
  <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>$('#example').DataTable( {
          "footerCallback": function ( row, data, start, end, display ) {
          ...
          ...
          ...
          }
  });</code></pre>
  </div>
  </div>
  </p>
  
  <p>But he throws a mistake like that <a href="https://datatables.net/manual/tech-notes/3" rel="nofollow noreferrer">error</a> because it is already initialized</p>
  
  <p>How can I solve the problem, keeping both method "add" and "footerCallback?</p>