Allow TestCase users to customise their instance.

Users of InstanceTestCase and InstanceLiveServerTestCase might need to
be able to give options such as a label to the default instance set up
by InstanceTestCase, so pass the options in a class attribute.
Placate flake8.

Well mostly anyway - I thing the 80 character line looks worse split.
Version 0.8.

Restore use of include_package_data (removed in d8384b7c) so that
e.g. templates are installed with a pip install.