ENH Add support for project.toml

See discussion in #210

A possible example, corresponding to the docs would be:

[tool.nbqa.mutate]
black = true
isort = false

[tool.nbqa.addopts]
flake8 = [
    "--ignore W503",
    ]

[tool.nbqa.configs]
mypy = ".mypy.ini"

[MarcoGorelli]

I’m tempted to say that this should be setup.cfg instead so we don’t need to add toml as a dependency.

I’d even go as far as saying that we should only use setup.cfg and warn if an .nbqa.ini file is detected so that existing users (if they exist) won’t be caught off-guard

EDIT

I’d tried adding you for ideas, then realised I’d already done that

[s-weigand]

But adding a setup.cfg just for a tools config, would be similar to the custom config file approach.