Add support for configuration file for saving user preferences

[EndlessTrax]

Feature: Support configuration file for user preferences

Currently, users must provide options like username and output database path each time they invoke the CLI. Adding support for a configuration file will make the CLI more user-friendly and efficient, especially for repeat users.

Proposed Solution

• Allow reading defaults (username, output path, preferred chess site, etc.) from a config file (e.g. .pgn-to-sqlite.toml, .config/pgn-to-sqlite/config.toml, or similar)
• When running, the CLI should load options from the config file if present
• CLI arguments should override config values
• Document config file usage (supported keys, example) in the README
• Maintain backward compatibility (no config file needed, just overrides)

Benefits

• Improved user experience
• Less repetitive CLI usage for frequent users
• Easier sharing of environment/setup between platforms