[Feature] support for XDG environment variables

[andrewcrook]

Support to use the following environment XDG variables as base directories for .kit .kenv .knode

XDG-CONFIG-HOME
XDG-DATA-HOME
XDG_STATE_HOME
XDG_CACHE_HOME

XDG Spec

if not set then use

KIT
KIT_DOTENV_PATH
KNODE

if these are not set fallback to

use users home directory for .kit .kenv .knode

You may like to look at the XDG-Ninja project that helps users to organise and clean up their home folder's dot files and dot directories