Plugins can now be declared in the chart `config.plugins` array and will only be applied to the associated chart(s), after the globally registered plugins. Plugin specific options are now scoped under the `config.options.plugins` options. Hooks now receive the chart instance as first argument and the plugin options as last argument.
Add `beforeDatasetsUpdate` and `afterDatasetsUpdate` plugin notifications during the chart update. Plugins are able to cancel the datasets update by explicitly returning false to `beforeDatasetsUpdate`. For consistency, rename `(before|after)DatasetDraw` to `(before|after)DatasetsDraw`.
The plugins service now accepts an array of plugin instances to register or unregister (for consistency, renamed `Chart.plugins.remove` to `unregister`). Also added a few methods to manipulate registered plugins, such as `count`, `getAll` and `clear` (mainly used by our unit tests).
Change the plugin notification behavior: this method now returns false as soon as a plugin *explicitly* returns false, else returns true. Also, plugins are now called in their own scope (so remove the never used `scope` parameter).
Rename `Chart.pluginService` to `Chart.plugins` (so move the old Chart.plugins array as a private member of the service), and rename `notifyPlugins` to `notify` for consistency with other service methods.