Makes it easier to change company-backends retrospectively (with a hook
or setq-hook!). Also simplifies how backend defaults are stored (no more
of this :derived and :exact business).
Also updates unit tests.
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).
