Extend API for plugins to modify exclude

[asottile]

In GitLab by @Pequex on Mar 5, 2019, 10:26

As per https://gitlab.com/pycqa/flake8/issues/398#note_144810532.

I was thinking about creating a flake8-gitignore extension, which would read the Git ignored files and add them to the list in flake8's exclude. It seems there is no API to do that right now:

@Pequex it seems the only apis given to plugins for modifying existing options right now are extend_default_ignore, extend_default_select, and remove_from_default_ignore (nothing to do with exclude currently) -- that isn't to say it wouldn't be possible.

[asottile]

going to continue this in #1105