Add warning flags to command-line

[n-osborne]

The warning system for the qcheck-stm plugin can be a bit verbose, informing the user about each skipped clause even if there is another one that contains the information it were looking for.

I propose to have three verbosity modes:

1. error only (when ortac have not been able to generate anything)
2. warnings that impacts the generated artefact or test coverage (when a function has been skipped because next-state is impossible to generate, or because there is no sut type in the parameters).
3. extra verbosity, displaying every warnings possible even about skipped clauses.

2 could be the default, 1 a --quiet option and 3 a --verbose option.