Read `config.yaml` from XDG standard paths

[erhhung]

As more CLI apps are adopting the path conventions laid out by the XDG Base Directory Specification, it would be nice if config.yaml and other generated files like thread history are placed in these standard paths—or at least look for them in these paths before using ~/.chatgpt-cli/:

• config.yaml — use $XDG_CONFIG_HOME/chatgpt-cli/config.yaml (if XDG_CONFIG_HOME is NOT explicitly defined, use ~/.config/chatgpt-cli/config.yaml)
• history/default.json etc. — use $XDG_DATA_HOME/chatgpt-cli/history/default.json (if XDG_DATA_HOME is NOT explicitly defined, use ~/.local/share/chatgpt-cli/history/default.json)