fix(dialog): Fix dialog corners

The dialog styles added by @sophi-est weren't being loaded into the compiled stylesheet. I've added
them to the elements index file and moved the rounded corners styles into and `.is-rounded` modifier
class.

docs/dialogs.stories.js

@@ -8,8 +8,10 @@ stories.addDecorator(withKnobs);
 
 stories.add('dialog', () => {
   const open = boolean('open', true) ? 'open' : '';
+  const isRounded = boolean('is-rounded', false) ? 'is-rounded' : '';
+  const selectedClasses = [isRounded];
 
-  return `<dialog ${open}>
+  return `<dialog class="nes-dialog ${selectedClasses.join(' ')}" ${open}>
       <p class="title">Dialog</p>
       <p>Alert: this is a dialog.</p>
   </div>`;