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.
Ok I think I'll retract on this front.  The one thing I want to flag is that since we potentially provide the user with a circle, they likely want their circle to show up (or attach a tooltip or w/e) and because of this i think it makes sense to provide a default styling.  If you feel strongly that the circle shouldnt usually be shown, then I that's fine by me.