Add use-package-defaults

Add use-package-defaults

Previously, the :config, :ensure, and :pin keywords had default
values (dependent on the values of the use-package-always-ensure and
use-package-always-pin). This change allows the user to customize the
default values used for those keywords, and add default values for
their own keywords in a non-hacky way.

This functionality would be useful for (as an example) the
quelpa-use-package package, which needs to use an advice to override
the functionality of :ensure. The same problem prevents adding a
use-package-always-quelpa variable in any reasonable way, without a
way to customize the default values of keywords.