Add Expiration

[wsdt]

It would be amazing if cache-entries expire automatically after a specified duration.

[timfpark]

I would love to have a PR to do this. It would involve essentially adding tracking for the expiration time of all of the entries, tracking the nearest expiration, and holding a timeout to do the expiration.

[eubnara]

I made a PR(#25).
I'd appreciated if you guys review this.

[sagupta1001]

Could we release this please? Our project is using v 2.0.1 and would like to upgrade to a version that has expiration.