Add way to suppress React devtools in create-react-app

[rohannair]

In facebook/react#11448 @gaearon had offered up a Webpack solution to suppress React devtools. It would be really useful to have that with react-scripts without needing to eject.

[rohannair]

Bah nvm, just found this: https://twitter.com/dan_abramov/status/933362684942274560