Add --open flag to open browser after launching server

[jsuarezatzendesk]

I'd like to be able to launch a dynamodb-admin instance and then open the browser with a single command. Since the server needs to stay running, dynamodb-admin && open http://localhost:8001 doesn't work, since the first process doesn't end, and dynamodb-admin & open http://localhost:8001 (to launch the processes in parallel) is not a good solution because the server isn't up by the time the browser makes the request, loading an empty or error page.

A good solution would be the opener package. I'm happy to prepare a PR with this, but wanted to open an issue and run the idea by the maintainers first.

Thanks!