Expose newly_created on THREAD_CREATE event

[MaSp005]

The Discord API passes the property newly_created: true on a THREAD_CREATE gateway event. As of 0.18.0, this property is not exposed in the threadCreate event on Eris.Client.

Since the event is fired multiple times for one thread's creation, this property allows the application to distinguish the duplicate emitted events without having to manually track the IDs of created threads.

This could either be implemented by adopting and exposing said property in ThreadChannel#update or by passing it as another argument in the emitted event in Shard.js:2159.

[bsian03]

added to #1464