React-native-navigation: [v2][iOS] Add support for iOS System Icons

[RubenSandwich]

Add support for setting iOS system icons for Navigator and Tab Bar buttons.

Version 1 of RNN had support for setting iOS system icons for navigator buttons: systemItem, however, this feature seems to be missing from V2 of RNN.

It would also be beneficial if support for iOS system icons were supported for the tab bar as well.

The relevant iOS classes are: UIBarButtonItem.SystemItem & UITabBarItem.SystemItem.

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This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
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[stale[bot]]

The issue has been closed for inactivity.