Reconfigure babel & flow

Babel bug T7044 caused generated code to contain flow comments. That in turn triggered facebook/flow#1311 which:

1. Forced us to configure flow such that it would ignore lib and required mapping configuration so that test files would also type check.
2. Forced users that depend on this package to go through the same configuration hazards as above.

With this change additional babel plugin is used to really stripping out comments from output and there for avoid flow failures when dealing with lib which is no longer ignored & there for users don’t need additional configuration.