mark_entry_as_read(entry, read: bool = True)

[lemon24]

(found during #254 (comment))

It would be nice to allow users like the entry_dedupe plugin to call mark_entry_as_read(entry, read=read) instead of having to call mark_entry_as_read() or mark_entry_as_unread() depending on the variable.

We can keep mark_as_unread() as an alias to mark_entry_as_read(..., read=False).

That said, if Storage.mark_as_read() becomes stable, maybe we don't need to do this anymore.