(feature): allow multiple org-roam directories (#178)

Each org-roam-directory gets its own cache. One can override the `org-roam-directory` variable locally via dir-locals:

((org-mode . ((eval . (setq-local org-roam-directory (expand-file-name "./"))))))

org-roam-utils.el

@@ -160,6 +160,7 @@ ITEM is of the form: (:from from-path :to to-path :properties (:content preview-
             (puthash file title file-titles)))
         org-roam-files))
     (list
+     :directory dir
      :forward forward-links
      :backward backward-links
      :titles file-titles)))