Enable nf-bars-group to support stacked bar charts

[adimitri]

It would be great if we could pass an option to nf-bars-group that would change the behavior of nf-bars to stack the different bars within a gorup on top of each other.

{{#nf-bars-group stacked=true}}
  {{nf-bars data=someData yprop="firstprop" ... }}
  {{nf-bars data=someData yprop="otherprop" ... }}
{{/nf-bars-group}}

[benlesh]

This is a good idea. The current idiom I think should be nf-bars-stack to mirror nf-area-stack.

Requires some thought.