Added support for to and from heading.

Added support for to and from heading.

* ox-koma-letter.el: documentation.
(org-koma-letter-special-tags-in-letter): list of tags that
are typeset as part of the letter.
(org-koma-letter--determine-special-value): determine which
value to use (for to and from currentlly) if more than
candidate is available.  Which value is selected depends on
`org-koma-letter-prefer-special-headings'.
(org-koma-letter-prefer-special-headings): a boolean stating
whether to prefer headings with to/from or the #+X_ADDRESS.

The following letter now exports "as expected".  In particular when
special-headings is t heading values have presedence so the
from address would be ="Rasmus\nEmacs\nOrg-mode"= where the
=\n=s would be escaped.  If specia-headings is nil (default)
the from would be my-adr.  The same is true for TO address.

Here's the contents

* from :from:

   Rasmus
   Emacs
   Org-mode

On a technical note, note that the default value will be used if
neither #+X_ADDRESS or a heading is given.

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