Added new plugin Callback `afterScaleUpdate`

Following a Slack conversation tadd new Callback to allow updates to datasets config (ie borderColor) based on results of the calculated scale, A typical use case would be apply a Gradient fill.

On branch feature_New_plugin_interface_afterScaleUpdate
Changes to be committed:
	modified:   dist/Chart.bundle.js
	modified:   dist/Chart.bundle.min.js