Add flake8 plugin support #118
Description
Since we can't integrate flake8 right away, we can start by working with its plugins.
Flake8 plugins have a well-defined Python API. For example:
- https://github.com/adamchainz/flake8-tidy-imports/blob/master/flake8_tidy_imports.py
- https://github.com/adamchainz/flake8-comprehensions/blob/master/flake8_comprehensions.py
Fiasko should discover the plugins the same way flake8 does.