Executable listings

[Penaz91]

When it comes to engine-specific editions (Like the planned SFML edition, see #122) it would be nice to add executable listings of code.

Each language package would get bundled with a bunch of well-commented source code listings that can be compiled (if necessary) by the reader, as well as modified (by having separated variables that they can change, for instance to change the behaviour of a jump).

Eventually the engine-aware editions can refer to those executables via specific boxes.