Add countdown option to iOSMode types

[elsieclark]

Summary

The countdown mode is a valid iOS mode, but the string is not recognized as a valid option by Typescript, and should be added to the type.

To avoid throwing an error, I currently need to use something like mode={'countdown' as any}. This is not the intended flow.

Fixes #28

Test Plan

What's required for testing (prerequisites)?

What are the steps to reproduce (after prerequisites)?

Compatibility

OS	Implemented
iOS	✅❌
Android	✅❌

Checklist

• I have tested this on a device and a simulator
• I added the documentation in README.md
• I mentioned this change in CHANGELOG.md
• I updated the typed files (TS and Flow)
• I added a sample use of the API in the example project (example/App.js)

[vonovak]

thank you!