Declarative schema catalog, --emit schema contract, and conformance test #36

Allow the four Bolt connection options to fall back to standard environment
variables when the corresponding flag is omitted (an explicit flag wins). This
keeps the password out of shell history and the process list.

Scope:

• --neo4j-uri / NEO4J_URI
• --neo4j-user / NEO4J_USERNAME
• --neo4j-password / NEO4J_PASSWORD
• --neo4j-database / NEO4J_DATABASE