title: How to add dynamic class for formly-field based on controller property?
body: <p>I need to add conditional Class to Form Field based on control property.</p>
  
  <p>for Ex : i have in my controller</p>
  
  <pre> $scope.isIssueFixed = true; </pre>
  
  <p>and my formly object is -</p>
  
  <pre>
  $scope.formly = {
  className: 'col-xs-6',
  key: 'Name',
  type: 'input',
  templateOptions: {
  label: 'Common Device Name'
  }
  </pre>
  
  <p>Now how we can add a condition class to above formly field,
  $scope.isIssueFixed?'ok':'No'; in formly object?</p>