added info CLI command

Summary:
Many issues filed on Github are missing platform/toolchain version information. Others have different ways of writing it, and require the issue writer to look in multiple places for these versions.
Other CLI tools like Ionic have this function, and it's incredibly useful. Related to https://github.com/facebook/react-native/issues/14420

Run in terminal/command prompt `react-native info`

```
trevors-imac:AwesomeProject tabrindle$ react-native info
Versions:
  React Native:  1000.0.0
  OS:  macOS Sierra
  Node:  v6.10.3
  Yarn:  0.24.5
  npm:  5.0.0
  Xcode:  Xcode 8.3.3 Build version 8E3004b
```

- CLA signed ✅
- Verify functionality + implementation
Closes https://github.com/facebook/react-native/pull/14428

Differential Revision: D5392446

Pulled By: hramos

fbshipit-source-id: 460079f3860c0af1e0b77bf26552c26032e974be

package.json

@@ -166,6 +166,7 @@
     "create-react-class": "^15.5.2",
     "debug": "^2.2.0",
     "denodeify": "^1.2.1",
+    "envinfo": "^3.0.0",
     "errno": ">=0.1.1 <0.2.0-0",
     "event-target-shim": "^1.0.5",
     "fbjs": "0.8.12",