Markdown live preview

Currently when markdown is enabled in the editor, the user has to flip to preview mode in order to see how their marked-up text renders:


This dual-mode experience presents a few issues:
* It's an extra step/click to get to preview mode
* Many users continually flip back and forth between modes while editing
* (Subjective) Markdown markup doesn't look as nice as already-formatted text
* Users coming from more traditional editors (Notes.app, Word, etc) are not used to this
* The markup causes the user to lose focus on their content

So, my suggestion here is that we implement a live markdown preview where markup is rendered in real time. At the lowest levels, notes are still markdown...stored exactly as they are today. There are a few editors that already do this — a great example is [Typora](https://typora.io/).

Additionally, this new mode could be a global (and/or note-level) setting (on by default) that users can disable if they prefer the "classic" Simplenote editor experience.

The issue names no file, test, or entry point; start by locating the macOS editor and its existing preview-mode implementation. Done means markdown renders live while editing, notes remain stored as markdown, and the proposed setting can disable the behavior.

Written by the indexing model from the issue text.