Support vue custom blocks

[KaelWD]

vue-i18n supports either one block with all languages or a block for each language with a locale attribute.

<script></script>
<template></template>
<i18n>
{
  "en": {
    "hello": "hello world!"
  },
  "ja": {
    "hello": "こんにちは、世界！"
  }
}
</i18n>

<script></script>
<template></template>
<i18n locale="en">
{
  "hello": "hello world!"
}
</i18n>
<i18n locale="ja">
{
  "hello": "こんにちは、世界！"
}
</i18n>