Add the possibility to have a shell or list isort_command.

[rimvaliulin]

Why do you need a such functionality? You can easily config I sort with files.

[cyrille-co2ai]

I want to call isort from my poetry environment (so poetry run isort), so that it knows about the 3rd party libs I've installed there. I cannot put this in any in-project config file, since they will be read only once isort is started.