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.
Wait I thought this assumption isn't true (at least for LinePlots) since placing the points out of order for LinePlots still visualizes a valid visualization.