Add a column accessor option to the heatmap
Add a column accessor option to the heatmap

* Add another optional accessor to the heatmap for retrieving the
  column of a datum instead of relying on index
* Change the x-accessor to flow options.column into xScale
* Add an ID generated from the region and indicator for easy debugging

unicef/rhizome,unicef/polio,unicef/polio,unicef/polio,unicef/rhizome,unicef/rhizome,SeedScientific/polio,SeedScientific/polio,SeedScientific/polio,unicef/polio,unicef/rhizome,SeedScientific/polio,SeedScientific/polio