Substitutions: refactoring and availability for C++; misc minor changes
Protocol change:
- Substitutions defined by a cell are sent to jupylates by printing a json dict rather than through the cell's value
- The cell may print any number of such json dicts
- Refinement of the spec: the json dict should map strings to string, to be substituted. Building on top of that to substitute literals is responsability of the language helpers / author.
Misc:
- Some more documentation for the examples
- Fixed examples/04-guess_output_cpp.md
- Two additional examples