[html-formfu] Grouped options in Select or Radiogroup

[bits]

What's the YML syntax necessary to produce an option group in a Radiogroup
or Select?

For example, how would the following be written using a option groups
instead of the 00-valued items:

---
  elements:
    - type: Select
      name: month
      options:
      - [ 00, ----Q1---- ]
      - [ 01, January ]
      - [ 02, February ]
      - [ 03, March ]
      - [ 00, ----Q2---- ]
      - [ 04, April ]
      - [ 05, May ]
      - [ 06, June ]
      - [ 00, ----Q3---- ]
      - [ 07, July ]
      - [ 08, August ]
      - [ 09, September ]
      - [ 00, ----Q4---- ]
      - [ 10, October ]
      - [ 11, November ]
      - [ 12, December ]