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.