Ability to use shell

[rajivshah3]

We'd like to be able to use environment variables in our prepublish command, but this seems to require shell: true to be passed to execa.command(). Currently, we're using a workaround that sets shell: true when a | is found in the command. It would be great if we could enable it with something like this:

"prepublish": [
        {
          "command": "do-something --token $SOME_TOKEN",
          "shell": true
        },
      ],