geom_violin quantile lines respond to alpha

The lines drawn by draw_quantiles in geom_violin are affected by the alpha parameter, so they cannot be included along with a transparent fill. Maybe there is a reason, but it seems to me that they should behave like any other lines.

Simple example:

```
ggplot(mtcars, aes(factor(cyl), mpg)) +
  geom_violin(draw_quantiles = 0.5, alpha = 0)
```
