-auto-pin-rm option for ipfs name publish

Regarding the comments here: https://github.com/ipfs/faq/issues/16#issuecomment-301192403

Would be great to have an option to auto-remove pinned files from local storage, if they're not part of a directory anymore that has been content-edited & republished under the same ipns hash. (Obviously, this would also extend to publications under keys different from "self".)

Such an option might however need a bit of intelligence, i.e. it would also have to be determined if an ipfs hash that's not part of a revised ipns publication anymore, is still published under a different ipns hash (check across all keys); if it's still part of another ipns publication, that should override any auto-pin-rm option, and the individual file should stay pinned to local storage.