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.
There's a risk if this gets executed before projectors get set up; however, an end-user should never have that happen, because the plot won't even draw without projectors.