Add all the customization magic system (courtesy of JWG) using `initsplit'

Install `initsplit-customizations-alist' defaults to correctly "route"
 - mu4e in mu4e-settings custom file
 - gnus in gnus-settings
 - org-mode in org-settings
 - emacs in default custom file (as a catch-all target)

Thank you John for such a great idea ! M-x customize is now a real pleasure and no longer a mess.

Add company-mode module