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

mypy_django_plugin/lib/helpers.py

@@ -54,6 +54,10 @@ class IncompleteDefnException(Exception):
     pass
 
 
+def is_toml(filename: str) -> bool:
+    return filename.lower().endswith(".toml")
+
+
 def lookup_fully_qualified_sym(fullname: str, all_modules: Dict[str, MypyFile]) -> Optional[SymbolTableNode]:
     if "." not in fullname:
         return None