Added Interaction.Hover.

Interaction.Hover acts on any Component that has implemented the
Hoverable interface, which allows the Interaction.Hover to query
the Hoverable for data given a cursor position.

Added a hoverMode() call to AbstractBarPlot, which will allow
Interaction.Hover to replace the Interaction.BarHover.
jsdoc this interface so its clear what this is for
maybe hoverOver and hoverOut can be made clear that it operates on whole hitbox b/c it sounds like its for individual bars to a naive reader 
hoverOverComponent?