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.
So as of the current state, lineplots with unsorted values are supported.  With this change, lineplots with unsorted values will be left in the dust for HoverInteraction.  Is this behavior we want?