Introduce a mechanism that enables choosing to use cppdef rather than cppexec

in cells which begins by hardcoded specific strings.

This has been tested successfully on the C++ training notebook dedicated to std::weak_ptr: adding one of two token to enforce cppdef in the cell that declares the forward declaration fixes the compilation error we got in a later cell.

Closes #13 (closed) and #7 (closed).

Also closes #10 (closed).