Support for defining enum array values with items.enum

This PR, when merged, will add support for defining allowed array values using its items.enum property. If items array exists and has exactly one element with the enum property, a list of strings specified under enum will be used to populate the allowed values list. Example usage:

multipleCheckbox:
  type: array
  title: Checkboxes
  items:
  - type: string
    enum:
    - Daily News
    - Promotion
  attrs:
    type: checkbox