Add support for per-point tooltip bkg colors.

if `point.point.options` is *always* defined, this could just be `var tipBackgroundColor = point.point.options.color || point.series.color`
but if `point.point.options` can be null, you'd want `var tipBackgroundColor = point.point.options ? point.point.options.color || point.series.color : point.series.color;`

or something like that.