Sync highlight between multiple bar charts

**WIP**

Demo [plunker](http://plnkr.co/edit/1Tj7kSFzsP1UQQ24zWsL?p=preview)

Related to https://github.com/dc-js/dc.js/issues/681

In current implementation, user needs to send an array, but maybe will be better if they just send a string and we could have central a registry for this groups.