title: Adding validations to rails aasm state
body: <p>in <a href="https://github.com/pluginaweek/state_machine" rel="noreferrer">state_machine</a> I used to do </p>
  
  <pre><code>state :cancelled do
    validates_presence_of :user
  end
  </code></pre>
  
  <p>it would automatically cancel the transition if user was not present. </p>
  
  <p>How do we add similar validations to specific states in <a href="https://github.com/aasm/aasm" rel="noreferrer">aasm</a>?</p>