Summary:
An attempt to address https://github.com/facebook/react-native/issues/9156.
cc grabbou Kureev
**Test plan (required)**
1. Added the following to the `package.json` of a `react-native-plugin`
```
"rnpm": {
"commands": {
"preunlink": "node node_modules/react-native-plugin/scripts/preunlink"
"postunlink": "node node_modules/react-native-plugin/scripts/postunlink"
}
}
```
2. Added files, `scripts/preunlink.js` and `scripts/postunlink.js` to the plugin. Each of them simply logs a string to the console.
3. Ran `react-native unlink react-native-plugin` to verify that those logs get printed.
Closes https://github.com/facebook/react-native/pull/9157
Differential Revision: D3749434
fbshipit-source-id: 40b94c9026db4f11e8f5be4a417a0670e8069be6
Geoffrey Goh