Add support for XDG base directory

[equt]

It should be reasonable to move the gi_list file into XDG_CACHE_HOME, and it seems to be currently hardcoded into the shell script.

I notice that the ignore script could also benefit from XDG specifications, so maybe it's worth making a quick walkthrough of all the scripts.

Commands

• git-ignore-io
• git-ignore

References:

• XDG Base Directory Specification

[equt]

Patterns which a user wants Git to ignore in all situations (e.g., backup or temporary files generated by the user’s editor of choice) generally go into a file specified by core.excludesFile in the user’s ~/.gitconfig. Its default value is $XDG_CONFIG_HOME/git/ignore. If $XDG_CONFIG_HOME is either not set or empty, $HOME/.config/git/ignore is used instead.

Quoted from Git SCM. Actually git ignore currently doesn't follow the default value and will throw an error on call.