title: validate a password with express-validator
body: <p>I'm using express-validator for express 3.x -- when the user changes their password or signs up for an new account, they have to enter their password twice.</p>
  
  <p>How would I write a custom validator that will push an error to the error stack in express-validator if the two passwords (two strings) do not match?</p>
  
  <p>Something like this:</p>
  
  <pre><code>req.assert('password1', 'Passwords do not match').isIdentical(password1, password2);
  var mappedErrors = req.validationErrors(true);
  </code></pre>