Testgenerator should not only generate testsuite and stub

On updating the exercise, the testgenerator should also update other files:

* [ ] `README.md`
* [ ] `rebar.config`
* [ ] `src/$slug.app.src`

On generating a new exercise, it shall also make sure that folders exist before attempting to create anything else:

* [ ] `$exercise` folder
* [ ] `src` folder
* [ ] `test` folder

Start by tracing the generator's exercise-update and new-exercise paths, then check how it creates the testsuite and stub. Done means updating README.md, rebar.config, and src/$slug.app.src, and ensuring the $exercise, src, and test folders exist before other output is created.

Written by the indexing model from the issue text.