Add a flush option before loaddata

[Wtower]

There are certain occasions that existing data needs to be flushed before loading data from fixtures.

Provide an option to flush data before loaddata:

./manage.py flush --noinput

[Wtower]

Additionally, make a note in README that when django-guardian and mysql is used, and loaddata auth fails because of

Could not load auth.User(pk=-1): (1062, "Duplicate entry 'AnonymousUser' for key 'username'")

Then edit record, change manually id to -1 and rerun deploy.

Further investigate the issue on #django.

[Wtower]

Problems with contenttypes when loading a fixture in Django

The answer is outdated, but using the latest option --natural-foreign seems to solve the minor issue from the comment above.