Introduce special face for TODAY in agenda
* lisp/org-agenda.el (org-agenda-today): New function.
(org-agenda-get-day-face): New function.
(org-timeline): Use org-agenda-today and org-agenda-get-day-face.
(org-agenda-list): Use org-agenda-today and org-agenda-get-day-face.
(org-todo-list): Use org-agenda-today.
(org-get-all-dates): Use org-agenda-today.
(org-agenda-day-face-function): New variable.
(org-agenda-get-day-face): Use org-agenda-day-face-function.
Signed-off-by: Julien Danjou <julien@danjou.info>