Make it possible to script lessons

[lefticus]

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We'll prefer ChaiScript (for obvious reasons) for the scripting technology.

The current interface to the lessons / goals can be found here:

https://github.com/lefticus/cpp_box/blob/2da3c85acbc64356af9a7a29a52e702c9f0c0308/src/cpp_box.cpp#L580-L608