Move all customization to project config

[RobertoRoos]

The CLI arguments for the sorter are nice, but there is no real reason to vary them within a project. It would make more sensor to safe these options to a project config file.

It could be a new TOML file, but there will always be a pyproject.toml too, so it makes sense to just add our config there.

We could also have a custom config file, e.g. tctools.toml, in case people don't place a pyproject.toml. The order of settings should then be read as:

1. tctools.toml
2. pyproject.toml
3. Other config, like .editorconfig for the formatter

To prevent confusion, options will not be merged. Instead only the first valid section is used.