Arbitrary key sorting for sorted charts

Pie, horizontal, and vertical charts can
be sorted by any property. Default is by value,
null sortKey will leave data unsorted. @azirbel @sreenath-addepar @Dylnuge 
Not sure how I'm supposed to update the docs.
This comment needs to be better; I think it could be replaced by a comment describing `SortableChartMixin`. Something like "this allows for data in charts to be organized based on a property of the data indicated by `sortKey`. For example, allows more control over the order of slices in pie charts or bars in bar charts". (That's just an idea; probably you can make a better comment)