Provide a initState builder helper

[pikax]

Since the initState should be usually based on the component props, it would be nice to have an out-of-box helper typed function

import { createInitState } from 'histoire/client'
import  MyComponent from './MyComponent.vue'

const initState = createInitState<typeof MyComponent>({ /* props */ })

Once vuejs/core#2693 is merged we can add that type into the slot:state

[Akryum]

Probably related to this issue: #31

[Akryum]

Automatic prop detection and controls are now implemented: #31 (comment)