Add SplitSummaryAndDocstringFormatter and update testutils

[DanielNoord]

The testutils update is very basic.

I think we would want pyproject.toml support as well, but for now just adding args in an .args file and separating them by spaces is good enough. Just make sure that when we start adding more and more of such files we start considering rewriting them so that we're not stuck with 100+ .args files when we finally get to it.

I have made this optional as I think this might be a bit opinionated and wonder the impact on projects like pylint.

Closes #25

[coveralls]

Pull Request Test Coverage Report for Build 1780549797

• 15 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 100.0%

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Totals
Change from base Build 1780538636:	0.3%
Covered Lines:	302
Relevant Lines:	302

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💛 - Coveralls

[DanielNoord]

Running over pylint gave incorrect changes. Turning this into draft.

[Pierre-Sassoulas]

LGTM but I think we could merge #32 before so it simplify the test.