Enable marking a chapter as a solution in the chapter itself

[jsorva]

Since #168, it’s been mark to configure a chapter as a model solution to a module. This is great. One suggestion for improvement, though: currently, the model solution is configured in the module’s rst file; at least in my case, it would be much more convenient to configure it in the model-solution-containing chapter’s rst file instead.

I do have a custom Sphinx extension (read: nasty hack) in place now for my own course, so I have things working there. But it wouldn’t suck to have this officially supported at some point in the future.