Plugin hooks and jsdoc enhancements

Make all `before` hooks cancellable (except `beforeInit`), meaning that if any plugin return explicitly `false`, the current action is not performed. Ensure that `init` hooks are called before `update` hooks and add associated calling order unit tests. Deprecate `Chart.PluginBase` in favor of `IPlugin` (no more an inheritable class) and document plugin hooks (also rename `extension` by `hook`).