Hi Orgsters,

Here is a patch to add support for using CUSTOM_ID properties for labels
and refs in the LaTeX exporter.

The patch uses the value of CUSTOM_ID when exporting a headline for the
associated \label, and when exporting a link for the associated \ref
command (or whatever).

For example, from this Org file:

===============================================================================
* Headline 1
  :PROPERTIES:
  :CUSTOM_ID: sec:headline1
  :END:
  Links to headlines which have no CUSTOM_ID still work normally, like
  this one: [[Headline 2]].

* Headline 2
  Links to headlines which have a CUSTOM_ID property will use this
  value to refer to them:

  This link refers to Headline 1 using the builtin syntax for
  CUSTOM_ID: [[#sec:headline1]].

  This one uses the fuzzy search on the headline text: [[Headline 1]].
===============================================================================

the relevant section is now exported as:

===============================================================================
\section{Headline 1}
\label{sec:headline1}
Links to headlines which have no CUSTOM\(_{\text{ID}}\) still work normally, like
this one: \ref{sec-2}.
\section{Headline 2}
\label{sec-2}
Links to headlines which have a CUSTOM\(_{\text{ID}}\) property will use this
value to refer to them:

This link refers to Headline 1 using the builtin syntax for
CUSTOM\(_{\text{ID}}\): \ref{sec:headline1}.

This one uses the fuzzy search on the headline text: \ref{sec:headline1}.
===============================================================================

Previously, the label for Headline 1 would have been \label{sec-1}, and
the links under Headline 2 would have been exported as \ref{sec-1}. 

This addresses an issue that was raised here, but got no response:
http://thread.gmane.org/gmane.emacs.orgmode/54039

I also need this behavior, which is why I wrote this.

Hope that's helpful!

Best,
Richard