title: Dynamic param value calculation depending on params in Paw
body: <p>I have some API that must be signed with request params hash.
  For example I have 2 params - login and password in request params. So I need to add the param checksum that is calculated with login and password fields hash.</p>
  
  <p>How can I implement it? Now when I try to calculate it, I have the self-dependency error.</p>
  
  <pre><code>login = test
  password = test
  somefield = lalala
  checksum = md5([login][password][somefield]) &lt;- here is dynamic evaluation
  </code></pre>