Implement export without DONE tasks
* doc/org.texi (Selective export): Document how to exclude DONE tasks
from export.
(Publishing options): Document the properties to be used to turn off
export of DONE tasks.
* lisp/org-ascii.el (org-export-as-ascii):
* lisp/org-docbook.el (org-export-as-docbook):
* lisp/org-html.el (org-export-as-html):
* lisp/org-latex.el (org-export-as-latex): Pass the :done-tasks property
to the export preprocessor.
* lisp/org-exp.el (org-export-with-done-tasks): New option.
(org-export-plist-vars): Add entry for :done-tasks.
(org-export-preprocess-string): Call `org-export-remove-done-tasks'.
(org-export-remove-done-tasks): New function.