Make zesje import free of side-effects

Closes #403 (closed)

This MR:

• makes zesje able to be imported without any side-effects.
• allows you to specify an app_config when creating an app

This is based on the approach in this article: https://medium.com/@frassetto.stefano/flask-celery-howto-d106958a15fe

For example, a zesje app can be started like this with a custom config:

from zesje import celery, factory

zesje_app = factory.create_app(celery=celery, app_config={some_variable: 'some_value'})
zesje_app.run()