keep examples canvas centered inside container

[helgasoft]

Added display:flex; justify-content:center; align-items:center; to #chart-container in example-bundle.js.

Currently command myChart.resize({width: V, height: V}); in Editor will show the chart non-centered: After this proposed change, resize display the chart in the center of the container: In addition to looking better, it'll also provide simple zooming for charts which do not support it now - like pie, sunburst, gauge, radar, sankey, etc.