conditionally set visit/login deadlines per login

It's fairly common for login pages to have a "keep me logged in" option. I'd love a way to do this with `actix_identity`.

Maybe a `Identity::login_with_deadline` function that would overwrite any visit/login deadline set in `IdentityMiddleware`? This seems difficult with the current setup, but maybe fields for per-user deadlines could be added to `Identity`? Then the existing `enabled` flags could be replaced with `Option` which is either a `Copy` of the `Duration` in `IdentityMiddleware` or a custom one, and that's what gets checked to see if a user should be logged out (not sure if that's even possible).

Probably easier would be to provide methods to access the `visit/login_deadline_enabled` fields on a given identity, but to achieve the "keep logged in functionality" you'd basically just have to choose between no deadline and a short auto-logout one (ideally we could keep a default deadline even if users choose to be kept logged in).

Would love to hear what others think!