add createdAt timestamp to _hoodie_config

[gr2m]

After #261 we have lost the information when an account has been started to use for the first time. For example, if app is used anonymously without signing up, there is no way anymore to find out when the user started to use it.

A use case would be that I'd like to show a different UI depending on whether the user has used the app before or not, e.g. show a welcome page or a tutorial for new users.

To get this information back, I'd suggest to add a createdAt timestamp, which would be generated and stored every time a new hoodieId gets generated.

[gr2m]

closing in favor of hoodiehq/hoodie-account-client#92 and hoodiehq/hoodie-account-server#154