Refactor to pass in configuration via a factory function

[weotch]

I'm imagining a new API that is like:

import { makeInViewportMixin } from 'vue-in-viewport-mixin'
export default {
  mixins: [
    makeInViewportMixin({
      once: true,
      intersectionObserverOptions: {
        root: => document.getElementById('scroller')
      }
    })
  ]
}