You can actually do that in Plunker right now by having an `index.md` file instead of `index.html` [1].
In the absence of an html index, the plunker preview server will try transpiling to html from different languages. Markdown is currently supported.
Also transpiled are things like less, scss, coffee-script and more. Essentially, your html requests a file with the target suffix (e.g.: .css) and the preview server will search for compatible source files to transpile to that target.
So, like a collaborative Markdown document editor, as well as a simple way to store those documents and choose them for reading/editing later.