Optimize which linters are run against the AST

[mschwager]

Currently, when running the flake8 plugin, we run every linter against the AST that flake8 passes us and flake8 does rule filtering of results based on what's enabled. So we run all linters despite what's set via --select or --ignore. Instead we should only run linters if they're enabled. This will speed up Dlint's run time.