title: Doctrine ORM Admin adding placeholder to datagrid filter
body: <p>Is there any way to add a place holder to a filter on the Sonata Doctrine ORM Admin?</p>
  
  <p>I'm trying to something like this:</p>
  
  <pre><code>protected function configureDatagridFilters(DatagridMapper $datagrid)
  {
      $datagrid-&gt;add('number', null, ['label' =&gt; 'User Pin', 
                                      'attr'  =&gt; [
                                             'placeholder' =&gt; '####',
                                             'title' =&gt; 'Enter the 4 digits pin'
                                             ]
                                     ]
                    );
  }
  </code></pre>
  
  <p>Note that this is just an excerpt of the working code.</p>
  
  <p>I'm expecting/needing to have a placeholder on the input field, and perhaps a tooltip with the title attribute.</p>
  
  <p>Is this possible? Hint?</p>