Add extensibility point on react-native-xcode.sh script

Summary:
Original pull request: #12863

Useful for generating source maps, for example.

Feature parity with Android, that already have this extensibility point on `build.gradle`:
```
project.ext.react = [
    extraPackagerArgs: [...]
]
```

Xcode > Build Phases > Bundle React Native code and Images
```
export NODE_BINARY=node
export EXTRA_PACKAGER_ARGS="--sourcemap-output ${SRCROOT}/build/main.jsbundle.map"
../node_modules/react-native/scripts/react-native-xcode.sh
```

[IOS] [FEATURE] [./react-native-xcode.sh] - Added $EXTRA_PACKAGER_ARGS extensibility point
Closes https://github.com/facebook/react-native/pull/16933

Differential Revision: D6771709

Pulled By: yungsters

fbshipit-source-id: a33614b45b1e3537daeb31504fdd3a402dc70ff0