Option to avoid the user account expiration by inactivity

Hi,

This PR add the option to use the expirable module without considering the users' inactivity days. 

Then, your app can consider that an user is expired only checking the `expired_at` attribute.

To setup this you just need to define `config.expired_after = nil` on the devise initializer.

ps: i didn't find any tests in the gem codebase, so i didn't do one for this patch