title: Validate several properties with conditions used FluentValidation
body: <p>I have class with <code>DateTime?</code> properties. In general they are pair (request/receive) of dates. I used <code>FluentValidation</code> in my project. Can I write validator which will receive two <code>DateTime?</code> params?
  Conditions:</p>
  
  <ol>
  <li>Request must be less or equal to Receive</li>
  <li>Receive can't be set when Request is null</li>
  </ol>
  
  <p>Probably I will have more conditions in future.</p>