Defaulted colors should be accessible from `chart.data.colors()`

Currently, there is no way to access the default colors.

Colors can be set two ways:
* Via `chart.data.colors` at generation time.
* Via `chart.data.colors` at runtime.

However if these are not provided, the chart intelligently picks some colors, but they are not accessible. 

This change populates the object returned by `chart.data.colors()` with the colors assigned. This way, the charts data is more transparently accessible.

Use case:
* Color selector widgets that allow column colors to be changed.
* Theming of external data. (Eg green data in the chart is also green on the table)