Add support for ignoring nested classes

[econchick]

Hey, I just made a Pull Request!

Description

Add support for ignoring nested classes with a new flag, -C / --ignore-nested-classes.

Motivation and Context

#65

Have you tested this? If so, how?

Checklist for PR author(s)

• Changes are covered by unit tests (no major decrease in code coverage %).
• All tests pass.
• Docstring coverage is 100% via tox -e docs or interrogate -c pyproject.toml (I mean, we should set a good example 😄).
• Updates to documentation:
  • Document any relevant additions/changes in README.rst.
  • Manually update both the README.rst and docs/index.rst for any new/changed CLI flags.
  • Any changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in the project's __init__.py file.

Release note