I'm not sure it'll be useful, but it seems nice to be able to set the
same values via config or environment variables. Perhap we should
simply use `config::Environment` to make everything configurable via
environment variables, but I'll leave that for later.
It's useful to have `signature()` live on `UserSettings` because that
will let us cache information (such as the timestamp) in the
instance. It will also make it easier to have the timestamp settable
via regular config files. I don't know that that will be useful, but
it seems like a clean way of implementing it if we can have
environment variables simply as an overlay of configs.
