Add hover interactivity to Plot.Line and Plot.Area.

Users can use `Interaction.Hover` with `Plot.Line` and `Plot.Area`. The `onHoverOver` callback will be passed the closest point to the cursor by X (Y breaks ties) within 15 pixels.

Close #848.
What if someone wanted the simple... 

highlight this selection onHover

While they could specify the radius at this point, I don't see why we don't start with a sensible default.