Provide an option to delete expired Blocked IPs

[richardm75]

Thanks for adding the ability to delete old "Failed Logins" records from the DB. (fix #37 - possibility to purge old entries). That's great.
Now the number of "expired Blocked IPs" is growing uncomfortably. Is it possible to add an option to purge those too? I have many hundreds that are up to 6 months old and I don't see the point in keeping them. Very recent entries might be interesting to look at in rare cases. But I don't see the point in maintaining the older ones, especially when they come from distributed attacks . I can't imagine when I'd want to review them individually.