ox-koma-letter: changed handling of subject.

ox-koma-letter: changed handling of subject.

* ox-koma-letter.el: =`org-koma-letter-subject-format'= can now be a list.  Default is t.
   If nil neither subject format nor type is exported.
* ox-koma-letter.el (org-koma-letter-template): better subject handling.
* ox-koma-letter.el (org-koma-letter-template): allow for t value of =#+OPTIONS: subject:VALUE=
* ox-koma-letter.el (org-koma-letter-template): export of subject format is independent
of title, but title is not independent of =:with-subject=.

The issue was with =#+OPTIONS: subject:t= one would get a LaTeX error.

New behavior:

If =#+OPTIONS: subject:(x,y)= then =\KOMAoption{subject}{x, y}=.

If =subject:x= then =\KOMAoption{subject}{x}=.

If =subject:t= then =\KOMAoption{subject}{...}= is not set but
\setkomavar{subject}{...} is set (in the output file)

If =subject:nil= neither are printed.

Signed-off-by: Alan Schmitt <alan.schmitt@polytechnique.org>