Pass debug flag into template (Nunjuck and filters)

#### Reason/Context
In the #363 debug flag was extended to provide more information, this is nice feature which could be pleasant in templates.
Pass debug flag in the way that it will be available in Nunjuck and so in filters.

#### Description
Maybe it's make sense to reserve parameter like `generator-debug` which will be `true` if you run generator with `--debug`. Not just `debug` because parameter with such name could be useful for template itself.

The issue names the generator's --debug entry point, Nunjuck template context, and filters, but no files or tests. Trace how --debug reaches template rendering and filter context, then verify the chosen exposed parameter is available in both templates and filters.

Written by the indexing model from the issue text.