Add dry-run mode

[esynr3z]

This mode is beneficial for any CI/automation over Corsair.

In this mode all inputs are validated and all files to be generated are printed in some structured format. No IO and actual generation.

This will allow user to use Corsair as linter for it's own inputs and inspect what outputs will be produced without touching anything on disk.

[esynr3z]

In v2.x Corsair can be used as linter for its own input files with corsair check command.
As for true dry run-mode, it is quite hard organize due to nature of generators and their extensibility. For example, how to be sure that user generator works in dry-run mode and avoids any IO?