Enabled UTF-8 when reading the configuration file

src/config.lisp

 (defun load-config (&optional (config-key ""))
   "Load the coleslaw configuration from DIR/.coleslawrc, using CONFIG-KEY
 if necessary. DIR is ~ by default."
-  (with-open-file (in (discover-config-path config-key))
+  (with-open-file (in (discover-config-path config-key) :external-format '(:utf-8))
     (let ((config-form (read in)))
       (if (symbolp (car config-form))
           ;; Single site config: ignore CONFIG-KEY.