Add a delete method for unsetting a cookie

Not sure if you want to leave the interface small but since it's non-obvious how to remove a cookie, I found it convenient to add this for my own use.

Another option would be to make it so if you did `cookie.set(<key>, null)` it would delete the cookie. The current behavior is that that will set the value of the cookie to `"null"` which is probably not what you want by calling `set` with `null`.