Add a method to VSlides so it can mount itself at the top level of the page with a mini-router

[wbillingsley]

With scala-cli, scripting decks is now so easy it might be common just to create a repository containing just a single deck.

In which, case being able to say

DeckBuilder(dimensions)
  .markdownSlides("etc")
  .veautifulSlide(content)
  .mountToBody()

would be nice and short.

This can generate a mini-router based on HistoryRouter, with the HomeRoute pointing to a vertical layout, and a route for each slide.

[wbillingsley]

Fixed in d384f99