Add as option the hover legend effect

http://jsfiddle.net/jbkunst/b3oywcsp/

In `hc_legend` I guess?

```css
/* Legend hover affects points and series */
g.highcharts-series,
.highcharts-point,
.highcharts-markers,
.highcharts-data-labels {
transition: opacity 250ms;
}

.highcharts-legend-series-active g.highcharts-series:not(.highcharts-series-hover),
.highcharts-legend-point-active .highcharts-point:not(.highcharts-point-hover),
.highcharts-legend-series-active .highcharts-markers:not(.highcharts-series-hover),
.highcharts-legend-series-active .highcharts-data-labels:not(.highcharts-series-hover) {
opacity: 0.2;
}
```

Use the linked JSFiddle as the behavioral reference and inspect the `hc_legend` configuration path. Determine how an option can expose the shown hover effect, then verify that enabling it dims non-hovered series, points, markers, and data labels while preserving hovered elements.

Written by the indexing model from the issue text.