Fixes #105

Now it also can be used like this:

import {AsyncComputedMixin} from 'vue-async-computed';

// my component
export default {
  mixins: [AsyncComputedMixin],
  data() {
    return {someValue: 123}
  },
  asyncComputed: {
    myAsyncValue() {/* ... */}
  }
}

Diff is pretty big, but it is because of whitespaces. In fact, there is only mixin moved to getAsyncComputedMixin and Vue.set replaced with vm.$set. So better to view with ignored whitespaces

Note: it may be breaking for users, who used it as const AsyncComputed = require('vue-async-computed') without interopRequire, because previously only default was exported and now both default and named are exported.