feat(avatars): add avatar component (#156)

docs/avatars.stories.js

@@ -0,0 +1,20 @@
+import { storiesOf } from '@storybook/html'; // eslint-disable-line import/no-extraneous-dependencies
+import { // eslint-disable-line import/no-extraneous-dependencies
+  withKnobs, radios, boolean,
+} from '@storybook/addon-knobs';
+
+const stories = storiesOf('Avatars', module);
+stories.addDecorator(withKnobs);
+
+stories.add('avatars', () => {
+  const options = radios('class', {
+    'is-small': 'is-small',
+    default: '',
+    'is-medium': 'is-medium',
+    'is-large': 'is-large',
+  }, '');
+  const isRounded = boolean('is-rounded', false) ? 'is-rounded' : '';
+  const selectedClasses = [isRounded, options].join(' ');
+
+  return `<img src="http://www.gravatar.com/avatar" class="nes-avatar ${selectedClasses}">`;
+});