transform both stroke/color properties on highlight

Ability to transform either the `stroke` or `color` when highlighting.

I use it roughly like this:

```js
var highlighter = new Rickshaw.Graph.Behavior.Series.Highlight({
  graph: graph,
  legend: legend,
  transform: function(isActive, series) {
    var obj = {};
    if (isActive) {
      obj.color  = "rgba(255, 0, 0, 0.5)";
      obj.stroke = "rgba(255, 0, 0, 0.5)";
    } else {
      obj.stroke = "rgba(0,0,0,0.1)";
      obj.color =  "rgba(0,0,0,0.05)";
    }
    return obj;
  }
});
```



It's a non-breaking as it has backwards compatibility with  `args.disabledColor`.