Add ActiveAdmin option to template (ctran#693)

ActiveAdmin defaults to false.  Add `active_admin` option so people using this gem can more easily enable annotating files in `app/admin`.

Author: danielricecodes

lib/generators/annotate/templates/auto_annotate_models.rake

@@ -7,6 +7,7 @@ if Rails.env.development?
     # You can override any of these by setting an environment variable of the
     # same name.
     Annotate.set_defaults(
+      'active_admin'                => 'false',
       'additional_file_patterns'    => [],
       'routes'                      => 'false',
       'models'                      => 'true',