Add undo/redo buttons #79
Description
As an optional extra control, using EditorState.undo / EditorState.redo (see https://github.com/draft-js-plugins/draft-js-plugins/tree/master/draft-js-undo-plugin).
We may want to refactor how custom controls are defined first, so we don't always add options on top of options and clutter the editor API / config.