Fixed #113: Added a setting variable that lets devs add extra paths to p...

https://github.com/bernardopires/django-tenant-schemas/issues/113

...ostgresql's search_path.

This way developers can install postgresql extensions in a dedicated schema and make it available globally.
Extensions can be installed to `public` schema too but the `public` is not always added to search_path, for example when running database migrations on --tenant apps.