Update index.d.ts (#1396)
Update index.d.ts (#1396)

* Update index.d.ts

Adding undefined as an acceptable return from a keyBindingFn to signify it has not been handled

* Update index.d.ts

Adding undefined to keyBindingFn in EditorPlugin
draft-js-plugins/draft-js-plugins,draft-js-plugins/draft-js-plugins,draft-js-plugins/draft-js-plugins,nikgraf/draft-js-plugin-editor,nikgraf/draft-js-plugin-editor,draft-js-plugins/draft-js-plugins,nikgraf/draft-js-plugin-editor