add method
Implementation
CountdownHandle add(CountdownOptions opts) {
final id = nextId();
enqueue(CountdownTask(
id: id,
total: opts.duration,
onUpdate: opts.onUpdate,
onComplete: opts.onComplete,
threshold: opts.threshold,
onThreshold: opts.onThreshold,
onReady: opts.onReady,
onStart: opts.onStart,
onCancel: opts.onCancel,
onPause: opts.onPause,
onResume: opts.onResume,
));
return CountdownHandle._(id, this);
}