text: Title: add GET /orgs/:org/outside_collaborators
  Body: Hi,
  
  Could you please add GET /orgs/:org/outside_collaborators ?  https://developer.github.com/v3/orgs/outside_collaborators/
  
  It's something like this in orgs.py:
  ```
  
      def outside_collaborators(self, filter=None, number=-1, etag=None):
          """Iterate over collaborators of this organization.
  
          :param str filter: (optional), filter members returned by this method.
              Can be one of: ``"2fa_disabled"``, ``"all",``. Default: ``"all"``.
              Filtering by ``"2fa_disabled"`` is only available for organization
              owners with private repositories.
          :param int number: (optional), number of members to return. Default:
              -1 will return all available.
          :param str etag: (optional), ETag from a previous request to the same
              endpoint
          :returns: generator of :class:`User <github3.users.User>`
          """
          headers = {}
          params = {}
          if filter in self.members_filters:
              params['filter'] = filter
          url = self._build_url('outside_collaborators', base_url=self._api)
          return self._iter(int(number), url, users.ShortUser, params=params,
                            etag=etag, headers=headers)
  
  ```