Example story with section/option groups

[techniq]

Would be nice to have an example story on how to group options into sections similar to this downshift example. Probably utilizing ListSubheader.

Need to think how windowing (with react-virtualized) will work with the headers. At first I considered passing items as:

{
  section1: [{ text: '1', value: 1 }, { text: '2', value: 2 }, { text: '3', value: 3 }],
  section2: [{ text: '3', value: 3 }, { text: '4', value: 4 }]
  ...
}

but I'm not sure how this will work with height calculations. We'll need to measure both the headings and the items, but not make the headings selectable (which only happens if you call downshift's getItemProps).