Pytest-structlog

- Better default behavior when we can detect that the code under test was using a [`PositionalArgumentsFormatter`](https://www.structlog.org/en/stable/api.html#structlog.stdlib.PositionalArgumentsFormatter).

- Adds a "captured structlog call" section in the test report in case of failed tests
- Reverts use of `structlog.reset_defaults()` in fixture setup. This caused issues in certain non-default configurations. Users should mock out their own structlog configure call in their test suite if necessary, with a pytest auto-use fixture.

- log capture will reset defaults in case user code has already configured structlog at library import time. closes https://github.com/wimglenn/pytest-structlog/issues/10

- Add basic usage notes for the fixture (which is displayed in `pytest --fixtures` output)
- Reset logging config on teardown (thanks karlb)

Initial release.