A command-line flag to sort __all__ re-exports

[parafoxia]

Generally speaking, imports in typed projects have a list (or tuple) of __all__ exports that should be made. #1358 introduces a way to do this using comments, but this can look quite messy and cluttered. For such a common use case, I would suggest a command-line flag that can automatically find and sort these __all__ exports.

Ideally this would look something like this:

isort . --sort-reexports

And in a config file (specifically pyproject.toml in this case):

[tool.isort]
sort_reexports = true

The name for the flag is taken from Mypy's "implicit re-exporting is disabled" message, but if there's anything more suitable obviously that should be used instead.

This should also support the notion described in #1815 when that's done.

[parafoxia]

I do plan to attempt to contribute this myself btw.