The tests show passing in an option `seed` to `create` -- ```js var alice = create({ seed: seeds.alice }) ``` https://github.com/ssb-js/secret-stack/blob/113efba0cfba73a6eebce8aa01b18d7d6ae5f3d2/test/server.js#L31 but I can't figure out what that is used for. Is it required?