Add extendSelect option to flake8 plugin

[Susensio]

--extend-select allows adding error codes without overriding the default --select

[ccordoba12]

Hey @Susensio, thanks for your contribution! It looks good to me, but there is one thing missing.

You need to run in the root of your clone the following command:

python scripts/jsonschema2md.py pylsp/config/schema.json CONFIGURATION.md

and then commit the changes that does to CONFIGURATION.md.

That's necessary because you added a new option to the server.

[Susensio]

My bad, I didn't read the docs till the end.
I've pushed a new commit with that fixed.

[ccordoba12]

Thanks @Susensio for your contribution!

Note: I fixed the errors reported in our Static code analysis workflow in PR #585.