Overview plugin

[janishutz]

I just had an idea to implement another plugin, namely one that can be used to automatically create an overview over your presentation (and maybe even unfolds a 3D-Presentation so it can be viewed in its entirety. That'd obviously be an optional data- attribute one has to set) and as another option have the slide's content be blurred and the slide's title be auto guessed and overlayed with a big font.

I'll start working on this plugin soon. Any thoughts/suggestions?

[janishutz]

Also, one should be able to specify the title with like a data-overview-title attribute

[janishutz]

Planning this for same PR as V3