v0.0.17

Changelog: v0.0.16...v0.0.17

• Added Executor.pendingPromise which holds the promise of the pending task execution, or null if there's no pending task execution.

• Executor.getOrDefault() can be called without an argument, default value is set to undefined in this case.

• Updated parallel-universe to fix off-by-one error triggered when a listener is unsubscribed during an event publish.