Make 'default-directory' safe for denote-directory

This relates to the example we cover in the manual, which was updated in
commit 47a9024.

Thanks to Sven Seebeck for reporting the bug in issue 16 over at the
GitHub mirror: <#16>.

denote.el

@@ -96,6 +96,8 @@ If you intend to reference this variable in Lisp, consider using
 the function `denote-directory' instead: it returns the path as a
 directory."
   :group 'denote
+  :safe (lambda (val)
+          (and (symbolp val) (eq val 'default-directory)))
   :type 'directory)
 
 (defcustom denote-known-keywords
@@ -230,7 +232,10 @@ We consider those characters illigal for our purposes.")
 
 (defun denote-directory ()
   "Return path of variable `denote-directory' as a proper directory."
-  (let ((path denote-directory))
+  (let ((path (or (buffer-local-value 'denote-directory (current-buffer))
+                  denote-directory)))
+    (when (and (symbolp path) (eq path 'default-directory))
+      (setq path (buffer-local-value 'default-directory (current-buffer))))
     (unless (file-directory-p path)
       (make-directory path t))
     (file-name-as-directory path)))