Hi,

This patch adds #+SUBTITLE to a couple of backends.  This property is
already supported in ox-texinfo and e.g. beamer.cls has a subtitles macro,
but ox-beamer.el has no #+SUBTITLE.  I have used subtitles in
e.g. applications for research funds.

The value-added is twofold:

    - It adds a consistent way to have subtitle across backends.  I'm
      explicitly assuming this is nice, but perhaps this is false.
    - Currently, it is, I believe, impossible to hack-up a subtitle in at
      least ox-odt.el.

It's not documented yet as I want make sure that it's not an undesirable
feature before progressing further.

WDTY?

—Rasmus 

-- 
The right to be left alone is a human right