support custom positioners

[sgratzl]

chartjs-plugin-datalabels/src/positioners.js

Line 167 in 61e3890

export default {

specifies custom behavior for different chart.js elements. I'm developing a bunch of chartjs plugins e.g., https://github.com/sgratzl/chartjs-chart-geo and it would be great if I can extent the list of custom positioners for new chart.js elements.