Support XDG

[chulkilee]

I can achieve XDG Base Directory that by setting KERL_CONFIG and KERL_BASE_DIR - but it would be nice kerl supports it out of box, like git does out-of-box.

Here is untested snippet.

Current:

KERL_BASE_DIR=${KERL_BASE_DIR:="$HOME"/.kerl}
KERL_CONFIG=${KERL_CONFIG:="$HOME"/.kerlrc}

New:

xdg_config_home="${XDG_CONFIG_HOME:-$HOME/.config}"
xdg_data_home="${XDG_DATA_HOME:-$HOME/.local/share}"

if [ -z "$KERL_CONFIG" ]; then
    if [ -f "$xdg_config_home"/kerl/kerlrc ]; then
        KERL_CONFIG="$xdg_config_home"/kerl/kerlrc
    elif [ -n "$HOME" ]; then
        KERL_CONFIG="$HOME"/.kerlrc
    else
        # shellcheck disable=SC2016
        echo 'Error: $HOME is empty or not set.' 1>&2
        exit 1
    fi
fi

if [ -z "$KERL_BASE_DIR" ]; then
    if [ -d "$xdg_data_home"/kerl ]; then
        KERL_BASE_DIR="$xdg_data_home"/kerl
    elif [ -n "$HOME" ]; then
        KERL_BASE_DIR="$HOME"/.kerl
    else
        # shellcheck disable=SC2016
        echo 'Error: $HOME is empty or not set.' 1>&2
        exit 1
    fi
fi

[rjcoelho]

It should be the default

[jadeallenx]

I like this idea but I don't have time to work on it right now. I would definitely be interested in a PR to implement it.