Storybook addons (#137)

* added addon-knobs to mediaPlayer

* Added a11y storybook addon

* added viewport addon

* added settings knobs

* added mediaUrl knob to timedTextEditor

* Added knobs for TimedTextEditor and TranscriptEditor component

Author: pietrop

.storybook/addons.js

@@ -1,3 +1,6 @@
+import '@storybook/addon-knobs/register';
+import '@storybook/addon-viewport/register';
 import '@storybook/addon-options/register';
 import '@storybook/addon-actions/register';
 import '@storybook/addon-links/register';
+import '@storybook/addon-a11y/register';

.storybook/config.js

@@ -1,4 +1,6 @@
-import { addParameters, configure } from '@storybook/react';
+import { configure, addDecorator, addParameters } from '@storybook/react';
+import { setOptions } from '@storybook/addon-options';
+import { withA11y } from '@storybook/addon-a11y';
 
 // automatically import all files ending in *.stories.js
 // https://webpack.js.org/guides/dependency-management/
@@ -12,6 +14,8 @@ function loadStories() {
   styles.keys().forEach((filename) => styles(filename));
 }
 
+addDecorator(withA11y);
+
 addParameters({
   options: {
     panelPosition: 'right',