color accessor and color scale for bubbleChart

[gordonwoodhull]

Currently it's selectedColor and unselectedColor, which is limiting - should be data-driven.

Could follow dc.scatterPlot or dc.bubbleChart or could even follow dc.graph and have cascading data-driven styles...

[MHeisoku]

I've just made a pull request which I think could address what you were looking for - at least it does for my use case. Just lifted some code from the choro implementation and tweaked unselectedColor