Enhancement/use xdg base dirs

[r900]

This adds the ability to use XDG_DATA_HOME for the data storage base dir, and XDG_CONFIG_HOME for the configuration base dir.

If not set it will fall back to ~/.local/share and ~/.config, as before.

(Also renames the variable STORAGE_DIR to CONFIG_DIR in config.py, for consistency :))

Tests:
When running env XDG_DATA_HOME=/tmp/gtd-data XDG_CONFIG_HOME=/tmp/gtd-config gentodo CMD, dirs and files were created in the defined places.
When the env vars are omitted it still uses the previous default and uses/creates the files there.

[csfore]

Looks good to me, thank you!