[O] Translate #+INDEX to Latex \index{} on export

[rpgoldman]

The following email contains a patch that will cause the latex export
preprocessor to rewrite #+INDEX: lines in an org file into LaTeX style
\index{} entries.

As far as I can tell, this should be The Right Thing --- latex doesn't seem
to do anything bad if there are index entries without makeindex.  So no
downside.  And it doesn't seem right to have to do #+INDEX for HTML export
and #+LATEX: \index{} for latex export.

I believe that this should be complemented by some additional code that
would:

1. Add code to turn on and off index generation and appropriately add the
makeindex package and the \makeindex

2. Possibly add code to turn /off/ this translation in the case where the
user wishes to have HTML and Latex documents indexed differently.

Cheers,
r