Fix handling of author and email in koma exporter.

Fix handling of author and email in koma exporter.

* ox-koma-letter.el (org-koma-letter-author): defaults to a function
that returns =`user-full-name'=.

Setting the variables `org-koma-letter-author' and
`org-koma-letter-email' to the values of `user-full-name' and
`user-mail-address' respectively, allows the user to skip =#+AUTHOR:=
and =#+EMAIL:= lines when configuring a letter.  However, if the user
wishes to set this information in LCO files, these variables need to
be set to nil.

With the old after-init-hook method my user name was always set to "".
Now org-koma-letter will (i) allow for default nil values (good if you
use LCO files); (ii) default to =`user-full-name'= and
=`user-mail-address'= like =ox-latex.el=.  These values are obtained
on-the-fly.

The two variables in question can also be strings or functions
returning strings.

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