+ [jsfm] supported computed options

[Jinjiang]

supported:

module.exports = {
  data: function () {return {x: 1, y: 2}}
  computed: {
    // single-way computed
    n: function () {return this.x + this.y},
    // two-way computed
    m: {
      get: function () {return this.x + this.y},
      set: function (v) {this.x = v}
    }
  }
}