title: UITextField attributedPlaceholder color and opacity in swift 4
body: <p>I have custom <code>textField</code> which has <code>@IBInspectable</code> property <code>placeHolderColor: UIColor</code> and it works fine. I set it by:</p>
  
  <pre><code>attributedPlaceholder = NSAttributedString(string: placeHolder, attributes:[NSAttributedStringKey.foregroundColor: placeHolderColor])
  </code></pre>
  
  <p>How can I set programmatically an opacity value for this property only, not for normal text in my textfield ? I didn't find any matching <code>NSAttributedStringKey</code> to do this</p>