A little hack to give subslide support to slidemode

I proposed a little modification (hack) to use **subslides** in this mode...
The idea behind this PR is that the notebook you had written (or will write) can be rendered by this live slidemode but **ALSO** by the reveal converter without any further modification in the ipynb source...
So, if you want to edit code live, you will probably use this live slidemode and you can now include subslides (it will be rendered as any other slide for now...), but if you only want to show some code and other interesting things in a nicer environment as the reveal slideshow provides **(;-))**, you do not have to do anything more than use the nbconvert...
Until we can come up with a total implementation of all the reveal converter features in this slide_mode, I think it is important to keep the use of both slideshows tightly joint... I mean it is important to have a **kind of subslide** implemented here...

Cheers.

Damián

pinging also @ellisonbg and @fperez, because you are probably using this slide_mode (and the reveal, I hope!! hehe).