Added filtering by category for org mode

[SidharthArya]

Added support for filtering by category in dashboard, so the users can now select the org categories which they wants to have in the dashboard.
The users would have to specify the category which they need in their dashboard, or if they don't all categories will be considered.

So in my config I have something like this:

(use-package dashboard
  :ensure t
  :config
(setq dashboard-items '((recents . 10) (bookmarks . 5) (agenda . 10)))
  (setq dashboard-org-agenda-categories '("Tasks"))
  (dashboard-setup-startup-hook))
 

[JesusMtnez]

Thank you for your PR! It looks good to me. But I am missing an update in README.md documenting the new feature 😉

[JesusMtnez]

Thank you @SidharthArya !