Theming solution

Currently it supports:

- provide/inject
- rootOptions
- vuex

```js
styled('div', (props, ctx) => {
  // ctx: the context of created functional component
  // so you can access:
  // ctx.injections.theme
  // ctx.parent.$root.$options
  // ctx.parent.$store
})
```

Not sure which one is the best though 😅