- `set -i` was replacing all fields of any existing item, losing tags, attachments,
publication date, etc. It now fetches the existing item if any, and starts from its
data. `-r/--replace` can be used to force a full replacement (and avoid fetching old
item).
- `--attachment` uses `action="extend"` so multiple `-a` can be used.
- Fix attachement data key (`url` -> `sources`).
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