Add "o" as overview navigation

[giflw]

This shows the overview slide, if provided

...

[henrikingo]

Ok, so this one I would like to approach completely differently. Instead of thinking of the overview slide as a special case, let's do something that achieves the same but in a more general purpose solution.

Create a completely new plugin, which we may call accesskey based on the similarly named HTML attribute. Then, modify the overview step like this:

<div id="overview" class="step" data-accesskey="o" data-x="..." data-y="...">

Then implement accesskey.js so that I can bind any letter to any slide I want.

You can, and should, modify the index.html demo and the presentations under examples so that their overview slide uses this feature.

[henrikingo]

I believe #852 implements this