Interfacing with the tour - user controls

[sa-lee]

From suggestions from @dicook and @uschiLaa

• allow half-range to be controlled by the user (as a slider, would there a be limit?) this would be really cool to have controlled via a bind event (https://vega.github.io/vega-lite/docs/bind.html)

• play / pause / restart: I like the idea of (shift)-click to pause / (shift)-click to play on the view. We can listen to events wtih vegawidget (https://vegawidget.github.io/vegawidget/reference/shiny-getters.html) . I think restart is trickier to implement but still doable via a button in shiny