title: Optional in FormDialog
body: <p>I'm using MS bot framework and FormDialog to ask the user to fill a form. 
  I would like some of the parameters to be optional and therefore added [Optional] indication to them:</p>
  
  <pre><code>[Prompt("Please specify if you have any additional requirements")]
  [Optional]
  string AdditionalRequirement { get; set; }
  </code></pre>
  
  <p>However, these questions still appear when the bot is filling up the form, and there is no way for the user to skip them (for instance just type "enter" or "skip")</p>
  
  <p>Is there anything I'm doing wrong or is this the expected behavior? Is it possible to have a parameter that the user can skip in case it is not necessary?</p>
  
  <hr>
  
  <p>update: seems the expected behaviour is that the optional parameters always show and there are certain words that can be understood as skipping the question, such as "No preference". </p>
  
  <p>An answer that would explain how to add "skip" as one of the words to skip the question would be accepted.</p>