Add unstable_enableLogBox

Summary:
This diff adds a new `unstable_enableLogBox` function to opt-into the new LogBox experience. If LogBox is not enabled early enough, we show an error with instructions.

With this, LogBox can be enabled with:

```
require('react-native').unstable_enableLogBox();
```

Changelog: [General] [Adds] unstable_enableLogBox

Reviewed By: zackargyle, rubennorte

Differential Revision: D18808940

fbshipit-source-id: 4b0234ddc4d1646515bf63110d5b02133780512e