Meet Doctor, a new React Native command

A react-native doctor is a new command to help you out with getting started, troubleshooting and automatically fixing errors with your development environment.  

• The doctor command currently supports most of the software and libraries that React Native relies on.
• Doctor will find issues with your development environment and give you the option to automatically fix them.
• If doctor is not able to fix an issue, it will display a message and a helpful link explaining how to fix it manually.