Allow Coalton config before Coalton is loaded

[stylewarning]

This PR allows configuration to reside in one's lisp config directory without relying on environment variables

my sbclrc has:

;;; Coalton configuration

(let ((config '((:compiler-mode          "development")
                (:print-unicode          t)
                (:perform-specialization t)
                (:perform-inlining       nil)
                (:emit-type-annotations  t)
                (:print-types            t))))
  (setf (symbol-plist ':coalton-config) nil)
  (loop :for (key value) :in config
        :do (setf (get ':coalton-config key) value)))

as my preferred default