org-export: Include title, author, date and email macros

org-export: Include title, author, date and email macros

* contrib/lisp/org-export.el (org-export-expand-macro): New function.
(org-export-as): Use new function.
* lisp/org.el (org-macro-expand, org-macro-replace-all): Change
  signature.  The function now accepts an alist of templates so it
  doesn't have to rely only on `org-macro-templates'.
(org-macro-initialize-templates): {{{date}}} is not anymore an alias
for {{{time}}}.  During export, it will provide the value stored in
DATE keyword instead.
* testing/lisp/test-org-export.el: Add tests.
* testing/lisp/test-org.el: Update tests.