Support dynamic perPage

Now you are able to use dynamic perPage, which means you can do something like this:

<input type="number" v-model="numberOfItemsPerPage">

<section v-paginate:numberOfItemsPerPage="langs">
  <ul>
    <li v-for="lang in langs">
      {{ lang }}
    </li>
  </ul>
</section>

vm = new Vue({
    el: '#app',
    data: {
      langs: [...],
      numberOfItemsPerPage: 3
    }
});