Geoffrey should print warning when source code comment is not used

[elfenpiff]

It can be helpful that geoffrey warns the user about unused geoffrey comments in the code. Something like this

int main() {
 //! [unused]
 //...
 //! [unused]
}

could lead to a warning like: the tag unused is not used in the documentation. This cleans up the code when we do not need the code snippet anymore or makes us aware that there is something missing in our documentation.

[elBoberido]

Good idea. I think I can add a command line option for this.