title: How to set indicatorType during setBackgroundImage for UIButton using Kingfisher
body: <p>In Kingfisher documentation there is information about setting indicatorType when set image for regular imageView:</p>
  
  <pre><code>    imageView.kf.indicatorType = .activity
      imageView.kf.setImage(with: url)
  </code></pre>
  
  <p>But how to set indicatorType when set background image for UIButton?</p>
  
  <pre><code>    myButton.kf.setBackgroundImage(with: url, for: .normal)
  </code></pre>