[react-native init] Add a flag to exclude Flipper installation

[o-alexandrov]

Related issues

• Question: How do i remove Flipper from React Native?
• Provide an easy way to install CocoaPods excluding Flipper (for release/CI)

Current problem

On iOS, there's a way to not install Flipper conditionally with NO_FLIPPER env variable, whereas on Android it's always installed as part of the debug release.
On Android, the only way to completely get rid of Flipper is to manually remove it from code.

Proposal

Projects that do not need Flipper would be cleaner (thus, simpler to understand/maintain), if users would have an option --no-flipper (to not have Flipper in their codebase).

$ npx react-native init APP_NAME --no-flipper

[thymikee]

@o-alexandrov please see this RFC: #641 and comment there if you see anything missing. Thanks!