skip_magic_trailing_comma = true config file support

[adamjstewart]

Is your feature request related to a problem? Please describe.

One of the reasons I use black is so that I can focus on code without having to focus on style. #1824 introduced a --skip-magic-trailing-comma flag to further control style. However, there's no way to set this flag in a config file, so the file format will change depending on whether or not a contributor uses the flag.

Describe the solution you'd like

I would like to be able to set this in a config file like pyproject.toml:

[tool.black]
skip_magic_trailing_comma = true

Describe alternatives you've considered

I could alias black to black --skip-magic-trailing-comma, but that only affects me, not other users who contribute to my library.

Additional context

@hauntsaninja opening an issue as you suggested. I dug around a bit in the code but couldn't figure out how config file parsing works.

[Jackenmen]

The above configuration works for me, are you using an up-to-date version of Black?

[adamjstewart]

Huh, I swear this wasn't working when I tried a month ago, but it's working now. Maybe I was using an older version of black. Thanks for your help!