Swap precedence of command_path and image_magick_path, add interpolations sugar

lib/paperclip.rb

@@ -71,10 +71,14 @@ module Paperclip
                                         "will be removed. Use :command_path "+
                                         "instead")
       end
-      path = [options[:image_magick_path] || options[:command_path], command].compact
+      path = [options[:command_path] || options[:image_magick_path], command].compact
       File.join(*path)
     end
 
+    def interpolates key, &block
+      Paperclip::Attachment.interpolations[key] = block
+    end
+
     # The run method takes a command to execute and a string of parameters
     # that get passed to it. The command is prefixed with the :command_path
     # option from Paperclip.options. If you have many commands to run and