Figure out how to pass a Node script to BROWSER

Created by: gaearon

We added support for BROWSER env variable that is interpreted by OS-specific command. On OS X, it runs open http://url.com -a <BROWSER>.

However I'm not sure how to make it execute a Node script, and if it's even possible. That would be the best for allowing use cases like #1096 and https://github.com/facebookincubator/create-react-app/issues/1450. It would be awesome to investigate how to get it working with a Node script (my naïve attempt at pointing it to a shell script with node env didn't seem to work), and maybe implement some custom handling for JS extension if we don't find a solution.