Add an external formatter API to be able to launch Run with custom formatters

[Pierre-Sassoulas]

Was necessary to tests the args parsing with the default value handled by something else than argparse. As we decided in #15 to let the coverage fall a little we could still use it to provide an API later on.

[coveralls]

Pull Request Test Coverage Report for Build 1671483567

• 8 of 8 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 100.0%

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Totals
Change from base Build 1671482333:	0.5%
Covered Lines:	219
Relevant Lines:	219

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💛 - Coveralls

[DanielNoord]

I'm converting this to draft as I wonder if during development towards 1.0 will find other necessary attributes and methods needed for formatters. For example, a way to set formatter specific options.

I think this would be a really good feature to put in 1.1. As in: make sure we get 1.0 to be compliant with both PEPs and that we are happy with how everything is set up, and then allow for some additional customisation with plugin formatters.