async computed property support

[LongTengDao]

What problem does this feature solve?

Redundant properties (and low cohesion for each) for asynchronous evaluation:
为了异步求值虚设条目（并降低每个功能的内聚性）：

<template>
  <div>{{ result1 }} {{ result2 }}</div>
</template>

<script>
export default {
  props: ["url1", "url2"],
  data () {
    return {
      result1: "",
      result2: ""
    };
  },
  watch: {
    async url1 (url1) {
      this.result1 = await getPromise(url1);
    },
    async url2 (url2) {
      this.result2 = await getPromise(url2);
    }
  }
};
</script>

What does the proposed API look like?

<template>
  <div>{{ result1 }} {{ result2 }}</div>
</template>

<script>
export default {
  props: ["url1", "url2"],
  computed: {
    result1 () {
      return getPromise(this.url1);
    },
    result2 () {
      return getPromise(this.url2);
    }
  }
};
</script>

[posva]

Duplicate of #4083