create a django-stubs compatibility plugin

[ghost]

closes #267

this creates a mypy plugin to add compatibility with the django-stubs package. currently, mypy will completely crash when attempting to run on a project using both django-stubs and django-configurations this is because configurations is never initialized by django-stubs

this plugin (when added to setup.cfg correctly) runs before the django-stubs plugin and initializes the importer correctly, allowing mypy to run

[ghost]

looks like the stubs team has a more permanent fix in the works here. i'll be closing this, and i've set up the plugin as a gist here in case anyone wants to use it as a workaround