Use flake8 config

[bobhy]

Substantial logic change for the addin; deprecate --flake8-ignore, --flake8-max-line-length and --flake8-max-complexity, instead lets flake8 configuration make these settings.

--flake8-ignore uses arguably better semantics than flake8 --per-file-ignores and --ignore but having them in the pytest configuration required duplicating them in the flake8 config if your IDE uses flake8, and per #66, seems to be a perennial point of user confusion.

Since this breaks backward compatibility, the version has been bumped to 2.0.0.

Also also fixes #66, because the plugin is now much less dependent on the (somewhat fluid) flake8 API.

[McSinyx]

+1 on the change, -1 on the random changes of style and default settings. I really want this supported, so please make this PR easier to review!