Use XDG_* environment variables to locate the config directory and data
directory. Uses ~/.config/zathura and ~/.local/share/zathura as fallback (the
directories specified in config.h's CONFIG_DIR and DATA_DIR respectively) if the
environment variables are not set.

Uncomment the ZATHURA_NO_XDG define in config.h to always use CONFIG_DIR and
DATA_DIR.

One can specify --config-dir and --data-dir to overwrite the precomputed config
and data location.