Use XDG paths for config files.

[jheretic]

One of the issues with running Murakami standalone from pip or from the git repository is that its default locations for configuration files and for dynamic configuration state are in system directories that aren't accessible by a non-admin user and/or don't exist. As a consequence, one always has to specify non-standard directories in order to run as a regular user. This pull requests shifts the default to use the Freedesktop.org XDG standard user directories for configuration and state, as discovered by the pyxdg package.

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This change is