ox-koma-letter: Export options to control letter

ox-koma-letter: Export options to control letter

  * ox-koma-letter.el ((org-koma-letter-use-subject):
  Configure printing of subject line.
  (org-koma-letter-use-backaddress): Configure presence of back
  address.
  (org-koma-letter-use-foldmarks): Configure presence of foldmarks.
  (org-koma-letter-use-phone): Configure printing of phone number.
  (org-koma-letter-use-email): Configure printing of email.
  (org-koma-letter-use-place): Configure printing of place.
  (koma-letter): Add export option to exporter backend.
  (org-koma-letter-template): Evaluate export options in
  exporter template.

Usage:

  #+OPTIONS: backaddress:t foldmarks:t subject:titled phone:t email:t place:t

Except for `subject', all options are boolean, i.e., the accept the
values `nil' and `t'. `subject' accepts the values defined in
`scrlttr2`, i.e., `afteropening', `beforeopening', `centered', `left',
`right', `titled', `underlined', and `untitled'.

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