Chooses the next value from the provided list of values (string-wise
comparison). Technically, the 'option!' syntax for toggling bools is now
redundant, but a translation from 'option!' to '--cycle option false
true' is kept for backwards compatibility.
The '--cycle' flag could also be technically optionally, since the only
thing that depends on it is preserving the error message for specifying
multiple values. (But I think it's best to keep it explicit, as a
principle-of-least-surprise thing)
Note: The business logic of picking the next value and setting it was
moved out to a separate function to avoid tripping pylint's
too-many-branches detector.
Fixes#47
