Modify kde alpha channel on plot.

I don't think it's possible to modify the alpha channel on kde plots. The alpha parameter only modifies the borders. It would be nice to be able to modify the alpha of the distributions.

Example code:
```
import numpy as np
import pandas as pd
import hvplot.pandas

size=400

y_target = np.random.normal(0,1.5,size)
y_predictions = np.random.normal(1,1,size)
performance = pd.DataFrame(list(zip(y_target,y_predictions)),columns=['y_target','y_predictions'])

performance.hvplot.kde(alpha=1) + performance.hvplot.kde(alpha=0.1)
```