title: Active Admin before filter for namespace only
body: <p>I have a namespace <code>admin</code>. I would like to apply a <code>before_filter</code> only to this namespace (not the default one, which is <code>superuser</code>).</p>
  
  <p>I can add the following code in <code>active_admin.rb</code></p>
  
  <pre><code>ns.before_filter :is_subscribed?
  </code></pre>
  
  <p>But then, the <code>superuser</code> gets the filter too.</p>
  
  <p>I tried adding the <code>before_filter</code> inside my namespace configuration :</p>
  
  <pre><code>config.namespace :admin do |ns|
    ...
    ns.before_filter :is_subscribed?
  end
  </code></pre>
  
  <p>but Rails throws an error : </p>
  
  <pre><code>undefined method `before_filter' for #&lt;ActiveAdmin::Namespace:0x007fa2e0f75f00&gt;
  </code></pre>