Add styling options to the Explorer

[maximlt]

The hvPlotExplorer only offers the alpha styling option. More options should be made available, that depend on the plot kind and on the plotting backend. Because there are so many styling options for e.g. Bokeh, it might be required to only offer a limited subset of them, the most useful ones.

[MarcSkovMadsen]

Consider whether its possible to set the initial styling options in the explorer constructor. That would also be helpful.

[maximlt]

Why?