Markdown Preview Plugin

It'd be cool to preview markdown in the editor window.

A considerable amount of software is developed on github and they have a standard of putting documentation in a README.md in the root of a project directory. It'd be cool to support this and sort them near the top of a folder in the folder browser. (I think gitlab does something similar).

It'd be nice to toggle this in the editor so you can switch quickly between doc reading mode and markdown editing mode.

We have a browser preview plugin, so maybe the web view code in that could be recycled somehow.