Added config to optionally disable deleting digitals on variant deletion

lib/spree/spree_digital_configuration.rb

@@ -1,6 +1,12 @@
 module Spree
   class SpreeDigitalConfiguration < Preferences::Configuration
+    # number of times a customer can download a digital file
     preference :authorized_clicks,  :integer, :default => 3
+    
+    # number of days after initial purchase the customer can download a file
     preference :authorized_days,    :integer, :default => 2
+
+    # should digitals be kept around after the associated product is destroyed
+    preference :keep_digitals,      :boolean, :default => false
   end
 end