viewMode=slideshow

This issue credit @wlach, who was motivated by [this post](https://medium.com/@mjspeck/presenting-code-using-jupyter-notebook-slides-a8a3c3b59d67) and observing PyData NYC presentations.

The concept is straightforward - it would be nice if we can also turn notebooks into slideshow presentations. We get at half of the heart of modern explanation - reports are valuable. So are slide decks, for better or for worse.

This could come in one of a few varieties:
- implement as some form of a plugin or custom stylesheet, so a user explicitly imports it
- implement it as a native presentational layer

Regardless of _how_ this gets achieved, the point is similar: slide decks are pretty important media for presenting information, and if I give you the slide deck, can I also give you the tools to reproduce the analysis? Something like this would allow iodide to expand the explanatory power of the platform.

re: setting the presentational layer, this might be a follow-on from having direct JSMD editing in-place. We will already want a way to embed code snippets & outputs into reports, something like:

```
%% md

# report and graphs

This is a report with some graphs and tables

%% js report-output report-input
var dataTable = rawData.map((d) => {...})
dataTable
```

This should in theory embed the variable `dataTable` into the report itself. If we take some kind of UX like that as a given, we could also have something like this:

```
%% md

# report and graphs

%% js slideshow-output
var dataTable = rawData.map((d) => {...})
dataTable
```

Or something like that. This idea could probably stand to be stress-tested.

The issue names no files, tests, or entry points. Start by reviewing the proposed JSMD editing and report-output concepts, then determine whether a plugin/custom stylesheet or native presentation layer fits; done means notebooks can be reproduced as slideshow presentations.

Written by the indexing model from the issue text.