Add a setting to decide when the posts of a user should be added to the site map

The module should have a setting to allow to decide the number of posts a user should create before his posts are showed in the site map.
This would allow to not include spam posts in the site map; it would be enough to set a high value for the default value (used for all the users who just registered) that would be overridden by the value specifically set for the user (when the administrator user decides the user can be trusted).