Core ML delegation on IOS

[kinivi]

In the new version of tflite it can be found the ability to delegate using Core ML. How the code can be changed to support it?
https://www.tensorflow.org/lite/performance/delegates

[lucyllewy]

See PR #188 for my attempt at solving this. I'd appreciate any testing and feedback you can provide to verify that it does what it's supposed to ready for @shaqian to merge into this repo.