* org-e-groff.el (org-e-groff-classes): Added
letter classes.
(org-e-groff-special-tags): New variable to identify special tags.
(org-e-groff--get-tagged-content): New function to retrieve
special tagged content.
(org-e-groff--mt-head): New function to create "memo" type headers.
(org-e-groff--letter-head): New function to create "letter" type headers.
(org-e-groff-template): Handle the "letter" type.
(org-e-groff-headline): handle special tags.