Add :authorizer connection option (#358)

[aaronrussell]

Allows a deny list to be applied to DBConnection managed connections. The authorizer is installed as the final step of connection setup so it does not interfere with the PRAGMA and load_extension statements issued while connecting.

{:ok, conn} = Exqlite.Connection.connect(
  database: "app.db",
  authorizer: [:attach, :detach]
)

• Docs added to Exqlite.Connection.connect/1
• 3 new tests
• Line added to changelog unreleased section