Add binary for cross-platform open from npm scripts

[stephen-fanfair]

This would be handy for teams like mine, working on a variety of platforms.

[bclinkinbeard]

+1 I want to be able to have the following npm script: "example": "st -d example -i index.html --no-cache && open http://localhost:1337"

[sindresorhus]

Check out opn if you need this.