Add pgpass support

- pgpass support respects the PGPASSFILE environment variable
- Reads from ~/.pgpass by default
Stub for DDL output
Add pg_graph_pgpass and pg_graph_ddl modules
Add optional writing of files, use pgpass

- Optionally save the graph data structure to disk
- Optionally save a dot file
- Use pg_graph_pgpass for getting the password from a pgpass file