Idea: Add global knobs to toggle/edit $vuetify props in stories

[morphatic]

In .storybook/addon-vuetify/decorator.js I added a knob to toggle the theme like so:

// inside of the `wrapper()` function
return Vue.extend({
  vuetify,
  components: { WrappedComponent },
  props: {
    dark: {
      default: boolean('Dark Theme', false), // or options.theme.dark see #2 
    },
  },
  watch: {
    dark (val) {
      this.$vuetify.theme.dark = val
    },
  },
  template: `
    <v-app>
      <v-container fluid>
        <wrapped-component />
      </v-container>
    </v-app>
  `
})

Now in all of my stories, there's a knob that allows me to toggle Vuetify's dark theme on and off to see what components look like in those scenarios. Here are some screenshots:

Dunno if this kinda thing should be baked into the addon or not, but I thought it was a neat trick that I'd share.