Provide an ARIA label, or other accessible version of chart data.

[UDAR-TomH]

First off, this is a great package, very useful. However, it would be even more useful if it automatically included an ARIA label that describes the datasets represented by the graph, or a fall-back table of the data. I am currently overriding the template for the chart block to do this manually (we have legal accessibility requirements), but having this implemented by default in the library itself would be a great help.