Hover tooltips

Stub — to be expanded

Opt-in interactive tooltips. Most meaningful on bokeh; matplotlib uses an annotation-on- hover engine.

bv.plot_scatter(x, y, hover=True)
bv.plot_scatter(x, y, hover={"label": "trial"})   # custom tooltip label

TODO: which plots are hoverable (HOVERABLE set); custom tooltip fields; mpl vs bokeh behaviour; pull from README §"Hover Tooltips".