Modify OpenBrowser.js to run node scripts specified with the BROWSER environment
variable . If the value of the BROWSER environment variable ends with '.js' a
child process is spawned to execute the script with node.js. Any
arguments passed to npm start are also passed to this script, as well as
the url where the app is served.
The command executed in the child process is:

node <pathToScript> [OPTIONS] <url>

Update User Guide.