Allow a custom URL to be opened

[exalted]

Open a specific URL instead of hard-coded / path.

For example, BROWSER_URL='http://localhost:3000/admin' react-scripts start, would open http://localhost:3000/admin in user's default browser, instead of http://localhost:3000/ (that is if custom PROTOCOL, HOST, and PORT are not provided).

This aligns perfectly, in my opinion, with existing behavior of opening with a different browser (i.e. BROWSER='google chrome' react-scripts start).

What do you think?

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[Timer]

Hi! We appreciate this pull request but opted to let users achieve this behavior via a script. You can set BROWSER to script.js and have it run that script for you. You can do anything you want in this script.

[exalted]

Thanks.