Improving the hvPlot UI

[philippjfr]

This PR is working towards getting the hvPlot UI into a state where we can release it.

• Separate DataFrame, Geometry and Gridded explorers while keeping hvPlotExplorer base class
• Expose all axis options
• Expose all style options
• Expose all geo options
• Expose all datashader options
• Expose all style mapping options

[coveralls]

Coverage decreased (-2.3%) to 66.521% when pulling a8e380b on ui_improvements into 00e1ceb on master.

[jbednar]

This code answers my immediate reaction to "Separate DataFrame, Geometry and Gridded explorers while keeping hvPlotExplorer base class", i.e., to ask for a factory that returns something of the correct type. One step ahead of me!

Still, if hvGeomExplorer is not a subclass of hvDataFrameExplorer, it seems problematic to switch between those two here. Conceptually, can't any dataframe become a geo dataframe by tacking on a geometry column? And so shouldn't the capabilities of hvGeomExplorer include all the capabilities of hvDataFrameExplorer, plus a few geo-related ones? I can't tell much from the implementations of these two classes about what their future is, but it seems at first glance that people could add functionality to hvDataFrameExplorer that would then not be available in hvGeomExplorer, which doesn't seem right to me.