Added a speaker deck showing next slide and notes

After the merge you can hit c character on your keyboard to make a clone window/tab.
The current tab will be used as speaker deck, containing the next slide + notes.

When notes are present add a slide:

```
[NOTE.speaker]
--
Note that the times are approximate. +
We want this class to be a conversation, not a lecture. So we might go longer in some parts, and spend less time on others, or just skip ahead entirely. +
In reality, we'll mostly try to stick to the flow of this deck. +
--
```