support attributes on the include level

e.g. Models.User.find({
    where: {'Users.id': id, 'Users.deletedAt': null},
    include: [
      { model: Models.X, attributes: ['id', 'url', 'thumbnailUrl',
'externalId'] },
      { model: Models.Y, attributes: ['id', 'url', 'thumbnailUrl',
'externalId'] }
    ]
  })