Use `opn` instead of `node-open` package

[SpaceK33z]

I'm submitting a feature request

webpack and webpack-dev-server version:
webpack: 2.1.0-beta.21
webpack-dev-server: 2.1.0-beta-.3

Please tell us about your environment:
OS X

Current behavior:

node-open is used when --open is passed in the CLI, to open a browser. This package has been unmaintained for two years.

Expected/desired behavior:

It should use opn, a better version of node-open.

What is the motivation / use case for changing the behavior?

opn contains a lot of fixes that node-open doesn't have, especially for Linux users.