Pins with expiry times

#### Version information: 0.4.11

#### Type: Feature / enhancement

#### Severity:

#### Description:

A few times I have wished that I could pin stuff (using the API, from a script that runs regularly), but only for a certain amount of time. This is useful for data which becomes less important after a certain amount of time to me. Without this feature, the client has to track pins in a database then remember to unpin over time.

It could potentially work like this:

ipfs pin $hash --hours=72

or

ipfs pin $hash --days=30

etc.

Start by reviewing the existing pin CLI and API entry points to understand how pins are created and removed. Define how duration options such as --hours=72 and --days=30 should be exposed and persisted, then verify that pins expire automatically at the requested time through both interfaces.

Written by the indexing model from the issue text.