Allow overriding :quelpa in init.before.local.el
Allow overriding :quelpa in init.before.local.el

This involved:
* Adding a new variable radian-quelpa-overrides.
* Adding functions radian-override-quelpa and
  radian-disable-quelpa-override to manipulate it.
* Adding another advice to use-package so that the value of
  radian-quelpa-overrides has an effect on where packages are loaded
  from.
* Specify the depths for the use-package advices explicitly, since we
  have more than one now.
* Moving the :demand keyword in the use-package declaration for
  quelpa-use-package to after the :quelpa keyword, because the advice
  that handles radian-quelpa-overrides is not too smart and can't
  handle :quelpa keywords that are not at the very beginning of their
  use-package declarations.

raxod502/radian,raxod502/radian