Support for contour plots

[cdeciampa]

I assume this is way down the pipeline, but support for contour plots would be great. I think it would have to do with the way the underlying polygons are treated (by combining them or something?). I took a swing at contours with holoviews' hv.Polygons and hv.Contours, but they were both the same result and only plotted the outlines of the polygons. This is supposed to be vertical wind shear contours overlaid on top of outgoing longwave radiation.

Additionally, it doesn't look like labeling contour plots is a feature supported by holoviews yet.