Support popular python library django-environ

It would be cool if this plugin supported `django-environ`, a popular library from PyPi.
The most notable difference is the API call to get a variable from a .env file:
Instead of `os.environ.get()`, one uses `env('VAR_NAME')`, where `env` is an instance of the class environ.Env .

See the example usage of the package [here](https://django-environ.readthedocs.io/en/latest/quickstart.html#usage).

I think a small addition to [PythonPsiHelper.java](https://github.com/adelf/idea-php-dotenv-plugin/blob/master/src/main/java/ru/adelf/idea/dotenv/python/PythonPsiHelper.java) is all that is needed to make it work.
I unfortunately don't have the setup to build and run IDE plugins, so I can't do it myself.