Cache and configuration directories now adhere to the XDG specification
by default. For example, the default cache location was changed from
``$HOME/.astropy/cache`` to ``$XDG_CACHE_HOME/astropy``, where
``XDG_CACHE_HOME`` itself defaults to ``$HOME/.cache``.

Affected functions from the ``astropy.config`` namespace:

- ``get_cache_dir``
- ``get_cache_dir_path``
- ``get_config_dir``
- ``get_config_dir_path``

In addition, temporary directories set through ``set_temp_cache``
or ``set_temp_config`` will now not be symlinked to default locations.
The new behavior is also meant as 100% cross platform: Windows isn't
special cased like it used to.
