Allow customizing the multiplex client file output filename.

[pataquets]

Addresses #489. Adds a new org-reveal-multiplex-client-filename custom variable. It allows to customize the multiplex client generated file name. Defaults to %n_client%e to preserve current behavior (reveal_client.html for a presentation configured reveal as file name). I'd prefer not to extend too much explaining its workings, as docstring aims to be self explanatory. Let's put it to test :)

I've not been able to add a file property to do it, but I think the solution is good enough as it's not a common setting to change out of the box. I can do it, if needed, with some advice.