Feature request: Support --enable-error-code to allow for optional checks

[RenDelaCruz]

It would be great to support mypy's --enable-error-code flag, to support some optional checks that don't have their own flag. Error codes: https://mypy.readthedocs.io/en/stable/error_code_list2.html#check-that-expression-is-redundant-redundant-expr.

This includes very useful error codes like:

• redundant-expr
• possibly-undefined
• truthy-bool
• etc.

[sobolevn]

I agree, this is a critical missing feature: we can't really create repros for optional checks via this amazing tool.

I will create a PR.