Suggested code for adding multiplexing

Hi, love org-reveal, but recently I've taken to using reveal.js multiplex support to enable my students to track my lectures on their laptops etc. I've been manually adding the necessary multiplexing option to the reveal initialise method, which is obviously a waste of time. So I've added a few options REVEL_MULTIPLEX... to automate the inclusion. 

Here is how it works (I've added some documentation to README.org so I'll be brief here).

1. add multiplex builtin
3. choose a socket.io server and get necessary details.
3. set the REVEAL_MULTIPLEX options
4. Generate as usual.

If those details are in place, the modified code will add master presentation multiplexing to the file.html. If it added those details, it will generate an additional file_client.html with client multiplexing initialisation in place.

To use, give your audience a link to the file_client.html and to present open file.html.

Everyone must have access to the socket.io server obviously, but assuming that, the audience will now track your slides.

hope this is useful. 

Must admit, although I've been programming a long time, I'm an ellipse noob, so apologies if the code lacks elegance. Once thing I couldn't see a way to avoid was the use of a defer client-multiplex variable that gets set to true on the first pass generation of the main html file, in order that this can trigger a subsequent second pass to generate the _client.html file if multiplexing is enabled. I'm sure there is a more elegant way. 

regards,
Stephen.