add DJANGO_CONFIGURATION_HOOK as optional parameter to allow customly…

[JulianFeinauer]

… configured settings instances

This is a first draft implementation for #1001 to have a starting point for a discussion.

See here for how this would be applied: https://github.com/pragmaticindustries/open-machine-plattform/tree/feature/new-launcher-with-pytest-fixes
Especially https://github.com/pragmaticindustries/open-machine-plattform/blob/feature/new-launcher-with-pytest-fixes/pytest_setup.py
which is the hook that would be called and would internally call settings.configure({...}) to prepare a valid settings singleton for later.

[NiklasMerz]

I guess you can close this now? ;-)

[JulianFeinauer]

I guess so : )