Support node scripts in BROWSER (facebook#1590)

* Support node scripts in BROWSER

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.

* Tweak code style

* Pin dep

* Comment out 0.10 docs

Author: GAumala

template/README.md

@@ -1560,7 +1560,7 @@ You can adjust various development and production settings by setting environmen
 
 Variable | Development | Production | Usage
 :--- | :---: | :---: | :---
-BROWSER | :white_check_mark: | :x: | By default, Create React App will open the default system browser, favoring Chrome on macOS. Specify a [browser](https://github.com/sindresorhus/opn#app) to override this behavior, or set it to `none` to disable it completely.
+BROWSER | :white_check_mark: | :x: | By default, Create React App will open the default system browser, favoring Chrome on macOS. Specify a [browser](https://github.com/sindresorhus/opn#app) to override this behavior, or set it to `none` to disable it completely. <!-- TODO: enable with 0.10: If you need to customize the way the browser is launched, you can specify a node script instead. Any arguments passed to `npm start` will also be passed to this script, and the url where your app is served will be the last argument. Your script's file name must have the `.js` extension. -->
 HOST | :white_check_mark: | :x: | By default, the development web server binds to `localhost`. You may use this variable to specify a different host.
 PORT | :white_check_mark: | :x: | By default, the development web server will attempt to listen on port 3000 or prompt you to attempt the next available port. You may use this variable to specify a different port.
 HTTPS | :white_check_mark: | :x: | When set to `true`, Create React App will run the development server in `https` mode.