Add appdirs.user_config_dir support.

In the latest appdirs builds, `user_data_dir`` has moved from
~/.config/spartan to ~/.local/share/spartan. A `user_config_dir` command
was added for the original behavior (~/.config/spartan).

If the user is using the latest appdirs (v1.3.0 and up), then use
`user_config_dir`; otherwise, fall back on `user_data_dir`.