Add `description` story option

[pocka]

close #106

This PR adds description story option which replaces the propsDescription. The intent of this change is 1) support for descriptions for slots and events 2) provide more clean API.

storiesOf(/* ... */)
  .add('foo', () => ({
    template: '<foo/>'
    description: {
      Foo: {
        props: {
          disabled: 'disabled prop'
        },
        events: {
          click: 'click event'
        },
        slots: {
          icon: 'icon slot'
        },
      }
    }
  }))

Live demo

[Xayer]

😍