Add visual style for selected dataset in the dataset list
Add visual style for selected dataset in the dataset list

@kanitw Good suggestion! I've settled on this for now; I'm going to
move on to another issue for the time being, but this does make it
much more evident which dataset is selected

![image](https://cloud.githubusercontent.com/assets/442115/10552636/3179ffae-7423-11e5-9817-2cc1bc1738e7.png)

uwdata/vega-lite-ui,uwdata/vega-lite-ui,uwdata/vega-lite-ui,vega/vega-lite-ui,vega/vega-lite-ui,vega/vega-lite-ui