Update Typescript Definition File closes #632 (#634)

This adds the `addDecorator` exported function to the definitions.

config/storybook.d.ts

@@ -9,6 +9,7 @@ interface Story {
     addDecorator (decorator: StoryDecorator): Story;
 }
 
+export function addDecorator(decorator: StoryDecorator): void;
 export function linkTo(name: string, ...params: any[]): void;
 export function storiesOf(name: string, module: any): Story;
 export function action(name: string, ...params: any[]): any;