Added support for custom Xcode Linker Flags in iOS Plugins

Hi, I've added support for custom Xcode Linker Flags in GC iOS Plugins (using the ios/config.json file).
I've reused the existing code for injecting Build Settings in the Xcode project, and tested everything by adding some existing plugins and building the resulting Xcode project.

Some native libs (like Google's AdMob library) need a special Linker Flag in Xcode, and this change adds support for linkerFlags in ios/config.json.
If you like the change, I will also add this feature to GC's documentation for Native iOS plugin development and send another pull request.

See my post on the game closure devkit forum:
https://groups.google.com/forum/#!topic/game-closure-devkit/VJ4UWiDgNnI

--Dion