Optional timeline element for scrolling through carousel.

[paytonshaltis]

Add an optional 'timeline' that shows the user's current progress through a carousel. Will need to keep the following in mind:

• Maybe let the user choose where to position this in relation to the Carousel.
• How to style the timeline.
• How the timeline should handle uneven scrolling positions (show 4, scroll 3, total 8). May just show the current page based on the original breakdown, and if there are more items on the screen from that fraction of the total items.