Update Vue.js examples to be elegant

[Hanks Zhang <zh...@gmail.com>]

I found the examples of Vue.js in our repo is outdated. Especially since we
upgrade the weex-vue-framework to 2.2.1, compile the examples will get the
warning as follow:

"component lists rendered with v-for should have explicit keys."

That's because of the Vue.js 2.2.0 [1] add this restriction: "When using
v-for with a component, a key is now required. You will likely see a bunch
of 'soft warnings' when you upgrade, but this does not affect the current
behavior of your app."

Moreover, there are many inappropriate usages in our examples. I think we
should update those examples to keep it efficient and elegant since
it's considered as official use cases.

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[1] https://github.com/vuejs/vue/releases/tag/v2.2.0
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Hanks