Refactor how company-backends are set and stored

Company backends are now built from an alist (+company-backend-alist),
which can be manipulated through set-company-backend!. Backends can now
be set to all children of a parent mode (text-mode, prog-mode, etc),
like so:

  (set-company-backend! :derived 'text-mode 'company-dabbrev)

or only for an exact major-mode:

  (set-company-backend! 'markdown-mode 'company-dabbrev-code)

Backends cascade. So combining the two examples above will cause
company-backends in a markdown-buffer (which is derived from text-mode)
to be (company-dabbrev-code company-dabbrev).

modules/completion/company/config.el

@@ -15,10 +15,9 @@
         company-frontends
         '(company-pseudo-tooltip-frontend
           company-echo-metadata-frontend)
-        company-backends
-        '((:separate company-capf company-yasnippet))
         company-transformers '(company-sort-by-occurrence))
   :config
+  (add-hook 'company-mode-hook #'+company|init-backends)
   (global-company-mode +1))