Add option for platform-specific code

[mih]

It would add options along the lines of content-<platform> and realcommand-<platform> to support platform specific documentation rendering and execution.

This, however, should be done together with improved support on the sphinx end. Ideally we would switch away from literalinclude and replace it with some kind of tabbed-viewer, where readers can flip though platforms (e.g. unix vs windows). Maybe something similar to the widgets used to render API docs for client software in multiple languages.

Here we could have Python, sh, cmd...