Adds support for pyproject.toml files (#639)

* Adds support for pyproject.toml files

Since mypy 0.900 the pyproject.toml files are supported.

This PR adds a support for it. It searchs for a `tool.django-stubs` section. This is an example configuration:

```
[tool.django-stubs]
django_settings_module = "config.settings.local"
```

Fixes #638

* Added TOML tests

* Use textwrap.dedent instead of trying to manually replace spaces

setup.py

@@ -26,6 +26,7 @@ dependencies = [
     "django",
     "django-stubs-ext",
     "types-pytz",
+    "toml",
 ]
 
 setup(