Associate all defcustoms to options in back-end definitions
* lisp/ox-ascii.el:
* lisp/ox-beamer.el:
* lisp/ox-html.el:
* lisp/ox-latex.el:
* lisp/ox-man.el:
* lisp/ox-md.el:
* lisp/ox-odt.el:
* lisp/ox-org.el:
* lisp/ox-texinfo.el: All defcustoms are defined as options in the
back-end.
* lisp/ox-icalendar.el (icalendar): Ditto. Also rename :with-vtodo
property to :icalendar-include-todo.
* doc/org.texi (Publishing options): Update back-end specific
properties. Remove @vindex entries so as not to clobber variables
index.
This change allows to change default value for any defcustom on the
fly when publishing. It also normalizes how to choose and name what
variables to define as back-end options.