Add support for the XDG Base Directory Specification

[SaraSmiseth]

This is a feature request for supporting the XDG Base Directory Specification.

The specification works around a bug during the early UNIX v2 rewrite which caused files prepended with a '.' to be ignored from the output of ls.
While this "bug" has become a feature for some, it has also become a headache for users when developers continue to assume HOME is a great place to dump configuration files and local caches.

To address these issues XDG Basedir was formed to give developers a standard location for these files and giving the users control over where they are placed in their HOME.

If you were to support the XDG specification the following locations would change:

Change ~/.saveddit/ to $XDG_CONFIG_HOME/saveddit and fall back to $HOME/.config/saveddit if XDG_CONFIG_HOME is not defined.