Add different styles for bivariate kdeplot

[mwaskom]

Two ideas:

• element="raster", using imshow or pcolormesh with the kde evaluation grid.
• element="scatter", which draws a scatterplot of x and y with the color of the points set by the KDE.

The latter would need some changes to the KDE estimator class, but those would be nice to have. (I think the idea would be KDE.transform(x, y) evaluates the density estimate on x, y rather than constructing a grid. If this method exists, it may be wise to move away from a __call__ interface for usual usage.