Viento.burst() "buffered" type support

[au5ton]

Currently, when running Viento.burst(), they must be "allAtOnce" or "oneAtATime", but what if you wanted a combination of the two?

For example, say you wanted the next element to be animated halfway through the animation of the previous? The result would be staggered or buffered, and could create a nice wavy kind of animation.

Let's implement it.