title: Time format in ActiveAdmin
body: <p>I have no idea how i can format time in ActiveAdmin.
  This is my index list:</p>
  
  <pre><code>  index do
      selectable_column
      column :book
      column :user
      column :time
      actions
    end
  </code></pre>
  
  <p>How can i format field :time as %H:%i:%s? Is it possible.
  I need something like:</p>
  
  <pre><code>:format =&gt; :short
  </code></pre>