Legend follows highlighted points

This pull request adds the ability to make legend follow highlighted points

![optimized](https://cloud.githubusercontent.com/assets/290451/2844508/4e3f2098-d08b-11e3-8d55-cc10fd30c59a.gif)

We're typically biased against adding new options to dygraphs, since it already has [so many of them](http://dygraphs.com/options.html). Would there be any loss of generality if this were incorporated into the `legend` option? Perhaps as `legend: "follow"`?