fix(ioslides): Shiny-specific initialization

[gadenbuie]

Fixes #2514
Fixes rstudio/shiny#3894

Currently, ioslides waits for the DOMContentLoaded event to initialize the slides, assuming that the <slides> markup will be there when that event happens.

Beacuse the slides are loaded via dynamic UI with uiOutput() (not just the slides, but the app content for any Shiny app in R Markdown), and due to recent changes in Shiny, the dynamic UI isn't present in the DOM yet. This is due to recent changes in Shiny that make dynamic UI rendered asynchronous.

This PR is a minimal fix that adds special initialization code for Shiny in the ioslides pandoc template and prevents the ioslides slide-deck.js from trying to initialize slides automatically in a shiny context.

This works because the slides markup, including the new initialization code, are all added to the page at the same time, so by the time loadDeck() is called we have a strong guarantee that <slides> is present.

cc @cderv @cpsievert (I can't request reviewers)

[cderv]

Ok good. this is a simple enough solution. That suppose to change the external deps slide-deck.js to add shiny specific thing, in two places. So maybe I'll move the loading in the template.

However, it seems the issue is that <slides> element is not there yet. So my first thought would have been to improve the defer code until the element exists.

Would that work too ?

I'll also wait for @cpsievert (or anyone else from shiny team) to review before merging.