Fix #588 - Refactor plugins.

- Added settings.py with PluginConfig.
- Rewrote jackaudio's load_init()
Although this is definitely out of the scope of this PR, I thought of a nice way to reimplement this function. Might be something to include in a future PR.

```python
import itertools
import os

def detect_devices():
    possible = ('/dev/fw{}'.format(i) for i in itertools.count(1))
    return list(itertools.takewhile(os.path.exists, possible))
```