Ability to add a member to a group, instead of specifying the complete list

On Windows, if you try to add a (domain) member to the local 'Administrators' group, you have to specify the complete list of members. Otherwise, puppet will remove current members that are not desired.

The `auth_membership` group parameter is designed to control whether the list of members is inclusive or explicit, like the `auth_membership` user parameter. However, the windows group provider does not take the `auth_membership` group parameter into account.