Provide an option to return only unread entries in a timeline

By default, a call to retrieve the timeline will still return already-read entries. However, in certain cases, such as if there are lots of unread entries and you start reading them from newest to oldest, or if somehow an entry was added far back in the timeline, you might want to see only the unread items.

This could be accomplished with an additional parameter to the [timeline](https://indieweb.org/Microsub-spec#Timelines) API call.

Start with the Microsub specification's Timelines API section linked in the issue and review how timeline retrieval currently describes read and unread entries. Define the additional parameter and its unread-only behavior, then update the specification so the API change and its expected result are explicit.

Written by the indexing model from the issue text.