Add --plugins command-line option

[GergelyKalmar]

Feature

It seems that currently the only way to make mypy load a plugin is to add it to the configuration file as plugins = .... I would love if it would be possible to invoke mypy in the command line with or without a given plugin enabled. For example:

mypy --plugins=mypy_django_plugin.main,other_plugin:custom_entry_point ...

Pitch

The main benefit of this feature is that it would allow us to dynamically load plugins when invoking mypy using mypy.api.run or when invoking mypy using the command line. Currently it is difficult to programmatically "inject" a plugin into a mypy run, this change would make this much easier.

[sobolevn]

This might be problematic, because, for example, our plugin (mypy-django) requires extra configuration in mypy.ini. Loading it dynamically would just fail without this config.

[GergelyKalmar]

That is true, however, configuration can also often be injected when necessary (see for example typeddjango/django-stubs#764 (comment) for one possibility with mypy-django). Also, configuration for "optional" plugins can still be kept in pyproject.toml, this would just allow one to choose during runtime whether to enable a given plugin or not.

[jp-larose]

Running into this issue right now where I want to specify a plugin in mypy.api.run, but it's not recognized.

[dibrinsofor]

Any work in this direction? I want to write tests with custom plugin options and this would be super helpful

[hauntsaninja]

PR welcome!