Add expireAt

[anthonyblond]

Related to #35

It is currently possible to set the (relative) TTL using expire(). It would be useful to be able to set it to an absolute time, calling redis.expireAt().

[guyroyse]

This would be very easy to add, I think. Might make a great first PR!

[garrett-green]

I know this was attempted last year but I have opened a fresh PR to add the feature.