add a way to externally reference a code snippet

[glyph]

Sometimes when I'm writing code for my blog, I'd like to make the code in question downloadable, and keep it in a separate file so I get nice syntax highlighting / snippets / etc when I'm working on it, and also so I can run it and write simple test cases for it to make sure it works.

Maybe markdown syntax that included a comment at the first line of the file, like this:

    ```python
    # from: foo.py
    ```

This could re-write the source file during the build process, to drop the contents foo.py into the document inline (indented as appropriate).

[lepture]

@glyph I am redesigning this software in the develop branch now. I will take this into consideration.