Fix #588 - Refactor plugins.

- Added settings.py with PluginConfig.
- Rewrote jackaudio's load_init()
If you're going to modify this, then you might as well change get rid of `n` and use `enumerate`.

```python
for i, device in enumerate(detect_devices()):
    ...
```

`n` -> `i`, and `i` -> `'device'.