allow to specify checker mixins

[ronny]

this would allow to do propper support for framework specific extra checks/bindings outside of pyflakes

for example, instead of the execnet support, i could pass in a mixin that overrides the check
or instead of the ErrorReturn check, that could also be a mixin

same goes for supporting __tracebackhide__

[nejucomo (nejucomo)]

I would like to argue against this kind of feature.

Plugins would make it less clear what it means when a codebase does or does not pass pyflakes. Furthermore, plugins may not uphold the Design Principles. https://github.com/pyflakes/pyflakes#design-principles

If a codebase needs framework specific checks, why not run pyflakes *and* a different tool on that codebase? Finally, other tools like pylint are highly customizeable and already have plugin APIs, so if you really want one tool that performs many codebase-specific or user-specific tests, maybe that tool is more appropriate.