Update export for tidier import functionality for front end frameworks

[ghost]

Chart.js/src/chart.js

Line 62 in a191921

module.exports = Chart;

currently this export type doesn't support ES6 import syntax that is documented in the documentation.
http://www.chartjs.org/docs/latest/getting-started/integration.html
import Chart from 'chart.js';
and requires import via import * as Chart from 'chart.js';
for universal importation across front end frame works
if an export default was substituted in stead of the module.exports then the documented es6 syntax would be correct.

[simonbrunel]

Duplicate of #5179