Specify an argument on the path to filter role results

Specify an argument on the path to filter role results, attached is my patch

Allows you to turn this option on in http://tribefly.dgtlmoon.localhost/admin/settings/site_user_list/roles (part of the site_user_list_roles module)

[ ] Allow the placement of a role name as a path argument to limit results

For example user might specify /site_user_list/editor, if "editor" which would restrict all entries to just "editor" role, you must ensure each of the possibilities is selected.