Modified navigateLeft() and NavigateRight() to include the current
vertical index, thus preventing errant left-right key presses from
resetting the vertical slide position.

This may cause unwanted behavior if sub-sections were used as is normal for reveal.js, but works well for all-vertical presentation formats, and quick testing revealed no obvious bugs.
Merge pull request #4 from BinaryPaean/arrow-hack

Fix for left/right arrows causing unwanted navigation