Summary:
NavigatorIOS supports four new properties:
- **rightButtonImageSource:** The source of an image to display in the top right. This must be a static image since UINavigationController only supports UIImages. Adding support for UIImageViews (or arbitrary views) is more complicated because custom views do not fade on touch and do not have hit slop the same way that UIImage buttons do. Usage: `rightButtonImageSource: ix('ImageName')`
- **backButtonImageSource:** Use a custom image for the back button. This does not replace the back caret (`<`) but instead replaces the text next to it.
- **leftButtonTitle**: Text for the left nav button, which supersedes the previous nav item's back button when specified. The main use case for this is your initial screen/UIVC which has nothing to go back to (since it is the first VC on the stack) but need to display a left button. This does hide the back button if there would have been one otherwise.
- **leftButtonImageSource:** Image source for the left button, super
Closes https://github.com/facebook/react-native/pull/263
Github Author: James Ide <ide@jameside.com>
Test Plan: Imported from GitHub, without a `Test Plan:` line.
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