With this new variable, user can load spacemacs anywhere, e.g.
"~/.emacs.d/spacemacs/". Only user's cache directory is still hard-coded
as "~/.emacs.d/.cache/". If user want to use spacemacs this way, drop
one line as the below in "~/.emacs.d/init.el":
(setq spacemacs-start-directory "~/.emacs.d/spacemacs/")
(load-file (concat spacemacs-start-directory "init.el"))