Automatic datashading/rasterizing

Right now, if someone attempts to plot something with a very large number of datapoints, the browser tab appears to lock up. It's not necessarily obvious to the user what the issue is, and they are likely to think that hvplot has crashed or has a bug.

We've previously discussed defining a size threshold above which plots get datashaded (or probably more appropriately, rasterized) automatically, but that was for HoloViews, where one would have to map the various HoloViews options (not easily accessible) into datashader function arguments. In hvPlot, such mapping is already being done from hvPlot arguments into both Datashader and HoloViews arguments, and so it seems like all we need to do is to change `datashade=True` into `datashade=1e7` and define some consistent measure of size (just length of dataframe, for Pandas?)?