Move company and auto-complete to a common layer.
They are not enabled globally anymore, each mode using them
must explicitly declare a hook.
Only one frontend is supported for a given mode, we have to
choose the best between the two.
Only one key binding to toggle auto-completion on `SPC t a` no
matter if it is company or auto-complete. The lighter in the
mode-line is Ⓐ for both frontends.
This is mostly a style thing, since I think it would probably be a lot
of effort getting spacemacs working on windows smoothly. But it is also
a first step towards a "try out spacemacs" script that doesn't require
someone to move their `~/.emacs.d` around.
