Timeline.js: start the timeline elsewhere than the title slide

[csbailey5t]

By default, timeline.js starts the timeline at the title slide or the earliest chronological slide. In a recept NeatlineTime update, we made it so that you could manually set the center date of the timeline. If we want to somewhat replicate that with timeline.js, we could take advantage of the start_at_slide configuration option, but would need to develop the interface to allow this to be set. https://timeline.knightlab.com/docs/options.html#start_at_slide