Use $XDG_CONFIG_HOME for config files

[tarqd]

I think it'd be better if linode-cli respected the $XDG_CONFIG_HOME variable and defaulted to $HOME/.config/linode-cli as the config dir if it's not set.

Personally, I don't like my home directory getting cluttered and this variable seems to be the standard for newer software unix-y systems.

Related package: https://pypi.org/project/xdg/

It's fairly small, just checks the env variables and has sane defaults if the variables are not set.

Reference: https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html