The `!ENV` tag is not usable in yaml

[thclark]

I'm not sure if this is specific to mkdocs, or part of teh yaml spec, but there seems to be a possibility for adding special tags in yaml for mkdocs configuration - particularly the !ENV tag.

When pre-commit is run we get the following error:

Check Yaml...............................................................Failed
- hook id: check-yaml
- exit code: 1

could not determine a constructor for the tag '!ENV'
  in "mkdocs.yml", line 3, column 11

Example yml:

site_name: Django Guardian
site_description: django-guardian - per object permissions for Django - Django Guardian
site_url: !ENV READTHEDOCS_CANONICAL_URL
repo_url: https://github.com/django-guardian/django-guardian

Our pre-commit config:

  - repo: https://github.com/macisamuele/language-formatters-pre-commit-hooks
    rev: 'v2.15.0'
    hooks:
      - id: pretty-format-yaml
        args: [--autofix, --indent, '2']

It would be helpful if we could define a custom tag (or if this one were incorporated).