[feat] Read configuration from XDG_CONFIG_HOME

As suggested by https://github.com/llimllib/gpt-bash-cli/issues/7 (also in Hacker News discussion), it's better to read configuration files from `XDG_CONFIG_HOME` to follow the [XDG Base Directory Specification ](https://specifications.freedesktop.org/basedir-spec/latest/). Currently ell is engaging `~/.ellrc` and `~/.ellrc.d`. This makes it difficult for users to manage their home directory. Adapting XDG Base Directory Specification will make it more organized. In the document, the default install path should be changed to `~/.local/lib/package/`, and a link to the entry script should be added to ` ~/.local/bin/`. This makes the installing process more complex but following contemporary standard. I'm not considering ell as a system wide application so it will not read from `XDG_CONFIG_DIRS`.