IPFS pin additions

Hi,

I'm not quite sure if i should post this here (js-ipfs) or here https://github.com/ipfs/ipfs as an api update.
Please do correct me if it doesn't belong here.

Yesterday i wrote a [Push notification API](https://github.com/ipfs/ipfs/issues/394) proposal. Again, for that one i'm also not sure if i posted it at the right location. In that proposal i'm suggesting a new pin option:
**ipfs pin --till-delivered**

Here i'd like to extend that to:
**ipfs pin --till-delivered --target \ --expire-in \**

- "--till-delivered" means the pin is alive till it has been received.
- "--target \" Required if "--till-delivered" is also set, as you then need to know who the intended target is. It's optional otherwise.
- "--expire-in \" (optional) If set, must always be in the future compared to the current date, end is being unpinned when the future date is reached. Or perhaps "--expire-at", it depends on if you want to say: "expires in 10 days" or "expires at november x 2xxx".

**Use case**
When having files that should be downloaded only once or shared for one specific target. File sharing services sometimes have an option like that to make a file available at X location for Y time for someone to download it. The same logic can be used for push notifications. The notification needs to be pinned till it has been received.

**To elaborate**
IPFS probably needs to have some mechanism that would allow a request like "Hey, i'm \, is there a new pin intended for me?". That would allow the target to handle the object. The "--expire-in" flag merely allows some automatic pin cleanup for when something has been received and is then not required to be available anymore. It can still be available, that's fine. It just doesn't "have" to be anymore.

I'm looking forward to hearing your thoughts.

Cheers,
Mark