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.
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.
