Have stories on each level of hierarchy

This is very asked improvement from the beginning of the hierarchy feature.

Today we have this:

storiesOf('Cells/Molecules', module)
  .add('with text', () => <Button>Hello Button</Button>)
  .add('with some emoji', () => <Button>😀 😎 👍 💯</Button>);

storiesOf('Cells/Molecules/Atoms', module)
  .add('with text2', () => <Button>Hello Button</Button>)
  .add('with some emoji2', () => <Button>😀 😎 👍 💯</Button>);

That will be shown as

- Cells
  - Molecules                
    - with text                
    - with some emoji   
  - Molecules               
    - Atoms                   
      - with text 2
      - with some emoji 2

Desired

- Cells
  - Molecules                
    - with text                
    - with some emoji   
    - Atoms                   
      - with text 2
      - with some emoji 2

In this case we should allow stories and groups to be in the same level.

[igor-dv]

I’ll take it 😉

[ndelangen]

This feature is complete and merged to the 3.3 branch