ipfs add --time-to-live

Sometimes, I'd like to add something but only for a specific amount of time. For example, if I'm adding mutable data that I'm sharing with IPNS, I'd like for the old data to expire eventually on my machine, being replaced by the newer data.

This could be useful for :

* Somewhat dynamic websites which don't need all the previous versions to be pinned.
* Package management mirrors generally host a manifest file with a list of available packages; while old packages remain, the package index should expire once the new one is added.