add basic support for bespoke.js

[mojavelinux]

• handles slides only, not bullets

[mojavelinux]

This is just a starting point. There is still work to do, but at least it drives the presentation.

One requirement is that the bespoke.js deck must be available via window.deck. Bespoke does not have a global registry to access decks it has setup, so this is an explicit requirement.

[melix]

Thanks. Could you update the README too?

[mojavelinux]

Ah, yes! I forgot about that.

After studying the architecture of bespoke.js a bit more, I realize that the correct way to get the reference to the presentation is to create a plugin that exposes the presentation to the global scope. That plugin can also be used to internalize things like disabling bullets and the like. I'm going to pursue that approach this week and see where it leads. We can still merge this as a first cut, but I definitely plan to take it deeper.