* *BREAKING CHANGES*
    * `onNotificationOpen()` renamed to `onMessageReceived()`
    * Changed key names for custom notification properties in FCM data messages to display system notifications in foreground.
    * Message payload is always delivered to `onMessageReceived()` for both data and notification messages.
    * `messageType` key indicates type of FCM message: `notification` or `data`
    * `tap` is only set when `messageType` is `notification`
    * Explicit dependency on `cordova-plugin-androidx-adapter` since Android implementation uses AndroidX so is incompatible with Android Support Library.
    * Set `remote-notification` background mode in native Xcode project for iOS.
    * Reworked plugin documentation.
* Support customisable display of system notifications while app is in foreground for both notification and data messages (both Android & iOS).
* Set default color accent and notification channel for FCM notifications.
* Add support for default and custom notification channels for Android 8+
    * Customise importance, visibility, LED light, badge number, notification sound and vibration pattern
* Calling `logError()` on Android now also logs to native logcat (as well as a non-fatal error to remote Crashlytics service).
* Fix `logError()` on iOS to log non-fatal error to remote Crashyltics service.
* Implement stubs for `hasPermission()` and `grantPermission()` on Android so they both return true in to the success callback.
* Rationalise permission check/request on iOS.
* Remove legacy support for iOS 9 and below.
* Support overridable default color accent for Android notification icons via `ANDROID_ICON_ACCENT` plugin variable.