Activating plugins

[andybak]

As far as I can tell the only way to activate any of the included plugins is:

1. Change 'active' to True in the plugin's source
2. Import it in the init py file

Is this correct? I propose the following:

1. Move the active flag for plugins into a settings file that can be overridden in the project's settings. The default settings should match the currently enabled plugins.
2. Automatically import all plugins in the directory.

This would allow projects to choose which plugins they should use. It would also allow external plugins without editing the source of django-test-utils. I don't mind submitting a PR but thought I'd run the general approach past people first.