Release Updates

It is desirable to be able to publish a release, and then later update that release with new content or fixes.

A simple, initial implementation could be to allow including an update pubkey in a release. If an update key is present, then later revisions could be signed with that key. Later revisions should include an update key, which may be the same, and after a user downloads a later revision, they should consider the new revision's update key to be the update key for the release, ignoring the old key.

More complex considerations include figuring out how to support git-like functionality that allows for efficient incremental update of releases.