Support thread-id available in iOS 12

[ken0nek]

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I agreed CLA.

Changes

• Add thread-id available from iOS 12

thread-id	String	Provide this key with a string value that represents the app-specific identifier for grouping notifications. If you provide a Notification Content app extension, you can use this value to group your notifications together. For local notifications, this key corresponds to the threadIdentifier property of the UNNotificationContent object.

I used https://github.com/RobotsAndPencils/buford v0.13.0.

References from Apple

• Using Grouped Notifications - WWDC 2018 - Videos - Apple Developer
• Local and Remote Notification Programming Guide: Payload Key Reference

[cubicdaiya]

Thank you for your contribution and I'm sorry for too late response. But long time has passed and we don't have the plan to incorporate this feature for now. So I'll close this PR. Sorry.