Fix #588 - Refactor plugins.

- Added settings.py with PluginConfig.
- Rewrote jackaudio's load_init()
The reason I added these default values is because there are various plugins (like the audio/video test sources) that don't use a Config at all. So if I remove these, I will need to add checks in the config loading methods to ensure we don't try to load a config for such a plugin.

Is that preferable to 'None' default values?