Add a flag to force create-react-app to use a specific package manager

Created by: leojiang

Background:

create-react-app uses yarn as the default installation tool if it is found in the environment. source code: line 195 in createReactApp.js

There're two special situation:

1. When using yarn, if the registry registry.yarnpkg.com cannot be reached with dns.lookup, then yarn will switch to offline mode and try to use local cache. But I don't have any cached modules at all. Then it goes to a dead end!! source code: line 616 in createReactApp.js

2. Anyway for situation no.1, uninstall yarn can solve the problem and npm is used instead. But I need to use npm and yarn both and I can't uninstall yarn. It goes to a dead end again!!!

So, Is it possible to add a parameter for user to specify which installation tool(yarn/npm) to be used dynamically?

Thanks!