Add Signet 0.4.0

Taken from [HubSpot/signet](https://github.com/HubSpot/signet/) [v0.4.0](https://github.com/HubSpot/signet/releases/tag/v0.4.0).

Requested by [HubSpot/signet#5](https://github.com/HubSpot/signet/issues/5).