Expose `flags.require` as config option

We can make it work very similar to usage with Liftoff by mirroring https://github.com/js-cli/js-liftoff/blob/master/index.js#L118-L125

The `requireLocal` method is exposed on the instance: https://github.com/js-cli/js-liftoff/blob/master/index.js#L28