org-export: Allow to toggle date insertion

org-export: Allow to toggle date insertion

* contrib/lisp/org-export.el (org-export-with-date): New variable.
(org-export-options-alist): Use new variable.
* contrib/lisp/org-e-ascii.el (org-e-ascii-template--document-title):
  Check `:with-date' property in communication channel.
* contrib/lisp/org-e-beamer.el (org-e-beamer-template): Check
  `:with-date' property in communication channel.
* contrib/lisp/org-e-groff.el (org-e-groff--mt-head): Check
  `:with-date' property in communication channel.
(org-e-groff-date-format): Remove variable.
* contrib/lisp/org-e-html.el (org-e-html--build-meta-info,
  org-e-html--build-preamble, org-e-html--build-postamble): Check
  `:with-date' property in communication channel.
(org-e-html-format-date): Remove variable.
* contrib/lisp/org-e-latex.el (org-e-latex-template): Check
  `:with-date' property in communication channel.
contrib/lisp/org-e-man.el: Do not redefine DATE keyword.