TODO: consider passing via Factory TODO: support per-hostname settings.

FlagErrorf returns a new FlagError that wraps an error produced by fmt.Errorf(format, args...).

FlagError returns a new FlagError that wraps the specified error.

NilBoolFlag defines a new flag with a bool pointer receiver.

NilStringFlag defines a new flag with a string pointer receiver.

StringEnumFlag defines a new string flag that only allows values listed in options.

CancelError signals user-initiated cancellation.

SilentError is an error that triggers exit code 1 without any error messaging.

A *FlagError indicates an error processing command-line flags or other arguments.