mark-as-read backfill

[lemon24]

It would be nice if the mark-as-read plugin would allow matching the current rules against existing entries in a feed, and not only new ones.

Two ways the user could do this, API-wise:

• Add a .reader.mark-as-read.once feed tag, similar to that used by the entry dedupe plugin. (suggested)
• Add a 'backfill': 'pending' field to the existing .reader.mark-as-read feed tag value / config, similar to readtime.
  • Unlike readtime, this would involve updating a user-provided value, so some care would be needed to clear the backfill field; #308 would help here.

[lemon24]

The .reader.mark-as-read.once version can be done by adding a new hook to before_feed_update_hooks that checks for the tag and then delegates to the existing _mark_as_read().