title: Nested form in active_admin with select or create option
body: <p>We are using active_admin for our administration backend. </p>
  
  <p>We have a model "App" that :belongs_to model "Publisher":</p>
  
  <pre><code>class App &lt; ActiveRecord::Base
    belongs_to :publisher
  end
  
  class Publisher &lt; ActiveRecord::Base
    has_many :apps
  end
  </code></pre>
  
  <p>When creating a new entry for the "App" model I want to have the option to either select an existing publisher or (if the publisher is not yet created) to create a new publisher in the same (nested) form (or at least without leaving the page).</p>
  
  <p>Is there a way to do this in active_admin?</p>
  
  <p>Here's what we have so far (in admin/app.rb):</p>
  
  <pre><code>form :html =&gt; { :enctype =&gt; "multipart/form-data" } do |f|
    f.inputs do
      f.input :title
      ...
    end
  
    f.inputs do
      f.semantic_fields_for :publisher do |p| # this is for has_many assocs, right?
        p.input :name
      end
    end
  
    f.buttons
  end
  </code></pre>
  
  <p>After hours of searching, I'd appreciate any hint... Thanks!</p>