Implement reStructuredText viewer

[chbrandt]

ReStructuredText documents are more powerful than Markdown files, links are more flexible and can be better defined (eg, footnotes). RST seems to be a better fit for Jupyter-Notebook "README" files (with instructions and external references about the environment), but JupyterLab does not have (yet, apparently) an extension for rendering (aka, a viewer) those documents.

Double check for the existence of such extension, if not, consider implementing one.