Change XDG base directory lookup

[zyrxadon]

Since version 2.0.0 detox searches for detoxrc in the directory $XDG_CONFIG_HOME/detox if the $XDG_CONFIG_HOME environment variable is not empty. However, the XDG Base Directory Specification states that

If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used.

Code should be added so that it looks up $HOME/.config/detox/detoxrc if $XDG_CONFIG_HOME is empty.