Continuous vertical scrolling

Added the ability to have the scrolling appear to be continuous - when you are at the last section and scroll down, the animation will continue downward but you will end up on the first section. It is similar when you are on the first section - scrolling up will cause the animation to continue upward but you will end up on the last section. Navigation is updated and hashes are maintained. This feature is very similar to the loopTop and loopBottom features but is targeted to make the end of a scroll (when there are many sections) not as jarring. 

Clicking on a navigation dot, or navigating to an anchor directly, will use the normal animation: scroll up if the section is a prior one, scroll down if the animation is a later one. 

And, just to be clear, this does not add horizontal scrolling (i.e. this is not a solution for issue 124).