config: remove useless battery argument

Refs #53
scripts: (volume) make it more flexible

- Dynamically choose main volume control
- Selectable volume adjustment step
- Selectable volume scale (percent/decibels)
- Allow capture devices
- Better option documentation

Refs #53
scripts: (memory) use instance

Refs #53
scripts: (disk) use instance

Refs #53
prepare 1.3