title: Marshmallow field set to another field's value if missing
body: <p>I have something like this:</p>
  
  <pre><code>class TimeoutSettings(StrictSchema):
      general_timeout = fields.Int(required=True)
      build_timeout = fields.Int(default='I donno')
  </code></pre>
  
  <p>Is there any way that I can set <code>build_timeout</code> to <code>general_timeout</code>, only when <code>build_timeout</code> itself is missing?</p>