Implement `init` command

[jamesaoverton]

This command initializes the SQLite database by creating three tables:

• ldtab: [key, value] pairs for tracking metadata
• prefix: [prefix, base] for tracking prefixes
• statement: [transaction, retraction, graph, subject, predicate, object, datatype, annotation] for statements (thick triples)

Later we will support other SQL databases.

The tests/init/* directory should have enough information for this.