Pgformat

- Release and document Docker images

- Add verbatim strings to PG format

- Warn when graph is reduced (at least in some cases)
- Fix bug in PG-JSON parser
- PG format: disallow unquoted identifier to start with opening bracket
- Use NULL byte as array delimiter in CSV export

- Disallow repeated edge identifiers

- Support directly writing into Neo4J
- PG format: require space between unquoted identifier and direction
- PG format: support edge identifiers

- Add experimental Mermaid export and --html filter