Summary:
Splits `ConfigT` into `ConfigT` (core + packager configuration) and `RNConfig` (RN CLI configuration).
Also guarantees that all values on `ConfigT` have a default when loading user configuration.
Reviewed By: jeanlauliac
Differential Revision: D4985965
fbshipit-source-id: bf036e22d6809e49746a9c3aa240ec403a085342