Add default known_only to parse_flags_with_usage

[sritchie]

This would allow users to take advantage of the error handling here while still being able to pass custom options on to FLAGS.

An alternative approach would just pass all kwargs along; but there's only one and this allows the docstring to stick here as well.

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[sritchie]

@googlebot I signed it!

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[yilei]

Makes sense. Could you please add a unit test, it will need similar setup as this one

[yilei]

(Actually make my reply a proper PR review)

Makes sense. Could you please add a unit test, it will need similar setup as this one