Unable to disable option Groups to support options with property "options"

This code assumes that if an Option has a options property then it MUST be a Group. In my use case, I have no need for a Group, but my option object does indeed have an options property, and so the Select assumes it has option groups anyways and excludes most of my options with empty options property.

It seems to me the solution probably involves one or more of the following:

1. Add a prop to Select to allow enabling of groups, e.g. isGrouping, isGroups, etc. Make that part of the condition here instead.
2. Add a prop to allow the user to configure the property that drives option groups, e.g. groupOptionsKey, or a functional version, e.g. getGroupOptions.

Example (notice "Ocean" is filtered out): https://codesandbox.io/s/option-groups-bug-s3pzm

If you guys agree with the issue and any of the solutions and need someone to make a PR, I'd be happy to if it helps. Thanks.

Start at the referenced Select.js line around option grouping and reproduce the behavior with the linked CodeSandbox, where "Ocean" is filtered out. Compare the proposed grouping-control and configurable-key approaches, then verify that options with an options property are retained when grouping is not intended.