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.
I think it makes more sense for this version to be as general as possible, and the `_doHover` version can pass in `this.closeDetectionRadius`, which is specific to hovering. Maybe I should rename `this.closeDetectionRadius` to `this.hoverDetectionRadius`?