Add method getTitle to get notification title in JS

[lukebars]

Summary

Currently, we can access notification's alertBody by using getAlert() or getMessage(), but there is no method for getting the alertTitlein JavaScript side. This PR adds that missing piece.

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)

[lukebars]

cc @Naturalclar

[Naturalclar]

@lukebars thanks so much <3