Add truncate method

lib/administrate/fields/markdown.rb

@@ -5,11 +5,19 @@ module Administrate
         renderer.render(data).html_safe
       end
 
+      def truncate
+        data.to_s[0...truncation_length]
+      end
+
       private
 
       def renderer
         Redcarpet::Markdown.new(Redcarpet::Render::HTML, extensions: {})
       end
+
+      def truncation_length
+        options.fetch(:truncate, 50)
+      end
     end
   end
 end