Allow hook_features_export_alter() to add attributes to exported user role Components

In another module's implementation of hook_features_export_alter(), I'd like to be able to add attributes to my exported user roles. But currently features.user.inc implements its own custom hook_features_export_render() which hardcodes a single value into the exported object.

The attached patch changes the structure of the $export array produced by user_role_features_export() so that the exported data is a keyed array instead of just a single string, and changes user_role_features_export_render() to render the array contents rather than creating its own single-value array inside. The actual rendered output of user_role_features_export_render() is not affected unless another module implements hook_features_export_alter() and changes the $export['features']['user_role'] array contents.