Use a 'BROWSER' environment variable with npm start to specify which
browser to open. if the value of 'BROWSER' is not valid executable file,
don't open any browser.

Running `create-react-app` in a Docker container causes an unhandled rejection error in nodejs > 6.5 because the `opn` module tries to open a web browser in a system that doesn't have one. I figured this error could be safely ignored.

A previous commit renamed the apple script to open chrome from
`chrome.applescript` to `openChrome.applescript`. That created
a minor bug. Even when chrome was open with the client app,
`npm start` would open the client app in a new Safari tab on
macOS instead of re-using the open tab in chrome.

* Extract some utilities into a separate package

* Add utils dir to `files` in package.json

* Do not create an empty `utils` dir on eject