Add support for setting Defaults

This is a simple change to support setting different Defaults for the users we manage sudoers rules for. I've used it like this on RHEL6 : 
```puppet
sudo::sudoers { 'wheel':
  comment  => 'Full root privileges for wheel group members (with password)',
  users    => [ '%wheel' ],
  runas    => [ 'root' ],
  defaults => [ 'env_keep += "SSH_AUTH_SOCK"' ],
}
```
That way, even after `sudo su`, the members of the `wheel` group can use git+ssh.