Support $XDG_CONFIG_HOME

Backwards compatible change that supports storing the bookmarks in
$XDG_CONFIG_HOME/cd-bookmarks.

* If the user as set the previously
existing $CD_BOOKMARK_FILE, this is always overriding.
* If the user has created the directory/file
  $XDG_CONFIG_HOME/cd-bookmarks/(bookmarks), use this
* Else fall back to old default $HOME/.cdbookmark

Fixes mollifier#3

cd-bookmark

@@ -17,8 +17,18 @@
 #    or: cd-bookmark [-l]
 #
 
+function _cdbookmark_get_config_file() {
+  if  ! [ -z ${CD_BOOKMARK_FILE+x} ]; then
+    echo "$CD_BOOKMARK_FILE"
+  elif [ -d ${XDG_CONFIG_HOME:-$HOME/.config}/cd-bookmark ]; then
+    echo "${XDG_CONFIG_HOME:-$HOME/.config}/cd-bookmark/bookmarks"
+  else
+    echo "${HOME}/.cdbookmark"
+  fi
+}
+
 typeset -r SCRIPT_NAME="cd-bookmark"
-typeset -r BOOKMARK_FILE="${CD_BOOKMARK_FILE:-${HOME}/.cdbookmark}"
+typeset -r BOOKMARK_FILE="$(_cdbookmark_get_config_file)"
 
 function _cdbookmark_print_usage() {
     cat << EOF