Use new start and stop plugin hooks

[stockiNail]

Going to beta7 of Chartjs, this version is introducing new hooks on plugin in order to manage better the life cycle of plugins.

I would suggest to move the logic

• from beforeInit and afterInit to start hook.
• from destroy to stop hook.

Doing that, you could add the feature to disable and enable the plugin at runtime, during chart lifecyle.

If it makes sense, I can propose the change by PR.

[benmccann]

changing the hooks used sounds fine to me