Add preserveVertIndex option


Hi, 
This patch allow horizontal transitions to the same vertical index in the next/prev stack by setting preserveVertIndex option to true. If the next stack does not have a slide at that index, the relevant direction arrow is disabled.

My partner is looking at using your reveal.js framework to do a "choose your own adventure" style interactive online story with illustrations. To do this requires the slides to transition to the same vertical index in neighbouring stacks, hence the patch :)
I've catered for both ltr and rtl orientations, and functionally everything works as expected. 
There are a couple of small niggles:
1. The transition effects aren't quite clean - they do an almost diagonal transition from source slide, through next horizontal slide at position[0] then to the vertical destination slide.
2. The overview mode still centres the vertical stacks without moving the surrounding stacks. Functionally this is still fine, but I would love to see the surrounding stacks all move in sync. There's a possibility of the stories getting quite large and complicated, and having the entire presentation moving as a unit would make creation/management a lot easier
I have a suspicion that these may be linked but they are both out of my league to resolve -

Thanks for considering my patch, I'm a novice javascript programmer, git user and very rusty tertiary qualified programmer so I apologise for any rookie mistakes!

Regards, 
Bernard Gray