typeProps (type) {
    const extract = obj => Object.keys(obj).reduce((props, slot) => {
      return Array.isArray(obj[slot]) ? props.concat(obj[slot]) : props
    }, [])
    const props = extract(this.options.slotProps)
    return this.options.library[type]
      ? props.concat(extract(this.options.library[type].slotProps || {}))
      : props
  }
Given a particular type, report any "additional" props to pass to the
various slots.
@param {string} type
@return {array}