[O] [patch, ox] Unnumbered headlines

[Rasmus]

Hi,

In a recent thread¹ Tom and Alan mention that authors sometimes need
unnumbered headlines, e.g. for prefaces.  This patch (tries to) add
this feature via the tag :nonumber: (customizable via Custom or
in-file).

I make two assumptions.  First, the tag is recursive, so if the parent
is not numbered the child is not numbered.  Secondly, I depart from
the LaTeX tradition of ignoring unnumbered headlines in the TOC
(except in the case of ox-latex.el where it depends on
org-latex-classes).  (See example below).

Needless to say such a feature needs to be discussed and I not sure
whether the greater Org community finds it useful or needless clutter.

In my opinion a :nonumber: tag is a natural continuation of :export:
and :noexport: and unlike :ignoreheading: the implementation is fairly
clean (or maybe I'm cheating myself here).  A reason for why to
include it is that it seems relatively easy to do *during* export, but
it's hard to consistently get it right on in both headlines and the
TOC via filters.

The patch is messing with ox.el, and thus I would appreciate a review
and potentially testing, in the case that it is agreed that such a
feature would be OK to add to ox.

It seems to work well with ox-latex.el, ox-ascii.el and ox-html.el.
It doesn't play well with ox-odt.el (headlines are still numbered).  I
will fix this as well as adding documentation if a consensus of the
worthwhileness of the patch can be reached.

Finally, here's an example output using ox-ascii

#+begin_src org
    * a (not numbered)                :nonum:
    ** aa (not numbert)
    * b (1)
    ** ba (not numbered)              :nonum:
    *** baa (not numbered)
    ** bb (1.1)

#+end_src

#+RESULTS: (TOC only, but the rest is as expected)
    a (not numbered)
    .. aa (not numbert)
    1 b (1)
    .. ba (not numbered)
    ..... baa (not numbered)
    .. 1.1 bb (1.1)


Thanks,
Rasmus

Footnotes:
¹   http://permalink.gmane.org/gmane.emacs.orgmode/89515

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0001-ox.el-Support-unnumbered-headlines-via-tag.patch
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