Array validations

• required: can't be undefined (default: false)

• sparse: can have undefined items (default: true)

• unique: can't have duplicate items (default: false)

• minLength: minimum quantity of items

• maxLength: maximum quantity of items

• exactLength: exact quantity of items

const Group = attributes({
  members: {
    type: Array,
    itemType: String,
    minLength: 2,
    maxLength: 5,
    sparse: false,
    unique: true
  },
  leaders: {
    type: Array,
    itemType: String,
    minLength: 1,
    maxLength: { attr: 'members' }
  }
})