Suppress browser launch only when BROWSER=none

Created by: CaryLandholt

As a follow-up to https://github.com/facebookincubator/create-react-app/commit/dc6edce99bc999252757114cb54721928abc28f1, the browser does not open when in a non-interactive shell (process.stdout.isTTY === false). This will happen when starting the CRA app from within another process, such as running a backend and front-end concurrently in development.

The browser should open regardless of interactive state; however, users can suppress browser launch when BROWSER=none (introduced in https://github.com/facebookincubator/create-react-app/pull/1247).

Browser does not open when app is started (via either npm start or yarn start).